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PREPARATION AND SPECTROSCOPIC CHARACTERIZATION

OF HIGHER OXIDATION STATES OF HEMOPROTEIN MODELS

By

Asaad Salehi

A DISSERTATION

Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1988

‘4)

0

(\l

567

ABSTRACT

PREPARATION AND SPECTROSCOPIC CHARACTERIZATION
OF HIGHER OXIDATION STATES OF HEMOPROTEIN MODELS

By

Asaad Salehi

This study utilizes synthetic octaethylporphyrin (OEP) complexes to
establish vibrational, electronic and structural characteristics of
hemoproteins' iron-porphyrin centers. The emphasis is primarily placed on
the ring-centered n-cation radical state which may be involved in the reaction
cycles of some hemoprotein enzymes and is also present in photosynthetic
chlorophylls. A systematic study of these model radicals should aid detection
and assignment of this particular mode of oxidation in the investigation of
hemoprotein catalytic cycles by Resonance Raman spectroscopy.

Metalloporphyrins (MP5) were oxidized by chemical methods using
oxidants such as AgClO4, Fe(CIO4)3, Brz and pheno+-sbc16' in
dichloromethane. The progress of the reaction was followed spectrophoto-
metrically until complete oxidation was achieved. The color changes
associated with radical formation are diagnostic: typically the initial pink or
orange color of the neutral MP changes to green or black upon oxidation. The
363.8 nm Ar ion laser is the principal excitation source in our RR
investigation as this wavelength is in resonance with the near-UV absorption
bands of the radicals and, consequently, it serves to minimize contributions
from the neutral porphyrin contaminants which absorb at longer

wavelengths. The vibrational modes mainly in the 1350-1700 cm'1 region are

Asaad Salehi

examined owing to their sensitivity to the metal oxidation, spin and
coordination states, and most importantly, they feature a linear correlation to
the macrocycle core-size.

Our RR data indicate that upon removal of an electron from the

highest filled porphyrin n-bonding orbitals, alu or a2u, the frequencies of
vibrations which are predominantly Cbe character increase dramatically (20-

30 cm’l); whereas, those with CaCm and CaN character decrease (3-10 cm'l) in

the RR spectra of OEP radical complexes relative to the neutral precursors.

These vibrational trends seem to hold in all the OEP complexes and is

consistent with 2A1u radical symmetry assignment. This observation,
however, contradicts the previous classification of CoIIIOEP+'2ClO4' as a
2A2u radical. Our data also suggest that the original classifica-tion of the

radical optical spectra into two types, believed to arise from 2A1u and 2A2u

ground states, may actually be caused by the planar vs. ruffled conformation
of the macrocycle at the oxidized level.

Metallochlorin (MC) n-cation radicals, on the other hand, feature a
more complex vibrational spectra in the high-frequency region owing to the
macrocycle reduced symmetry and increased conformational flexibility which
result in more RR-allowed bands and increased likelihood of changes in
mode composition, relative to their MP analogs. This is demonstrated by a
vibrational mode in the 1500 cm"1 region which increases in frequency upon

oxidation, in contrast with its MP counterpart. This characteristic is believed

to result from both Cbe and CaCm contribution to this mode in MCs which
differs from predominantly CaCm character of the corresponding mode in

MP5.

TO MY PARENTS

ii

ACKNOWLEDGEMENTS

I wish to express my deep appreciation to my advisors, Drs. Chris K.
Chang and Gerald T. Babcock for their guidance, encouragement and
friendship during the course of this work. I am also grateful to Drs. Michael
Rathke and Alexander Popov for serving on my committee. Financial
support provided by the Department of Chemistry at Michigan State
University is gratefully acknowledged.

Many thanks go to my good friend and colleague Tony Oertling whose
contribution to this study has been invaluable. I wish to thank all of the
members of Dr. Chang's and Dr. Babcock's group for their help, friendship
and support. I am especially grateful to Myoung S. Koo, Weishih Wu and
Juan L.-Garriga whose friendship in and out of the lab will always be
remembered; My special thanks go to a good friend, Mike Tornaritis, who has
been like a brother to me during the past few years. I appreciate the help of
Beverly Carnahan in typing my thesis and Ellen Rzepka for her sincerity and
help in keeping my academic records accurate.

Finally, I wish to thank my parents for their faith in me, their

continuous love, encouragement and support. I dedicate this work to them.

iii

TABLE OF CONTENTS

Page
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES .......................................................................................................... vi
Chapter 1 - INTRODUCTION
Research Objectives ........................................................................................... 4
Porphyrin n-cation Radicals .............................................................................. 6
Metal-porphyrin Interactions ............................................................................ 9
Site-Selective Oxidation: Metal vs. Porphyrin
Centered Oxidation Induction ..................................................................... 11
Porphyrin Peripheral Substituents ................................................................ 12
Metal Axial Ligand(s) ........................................................................................ 14
Solvent (or Medium) ........................................................................................ 16
Temperature ....................................................................................................... 17
Differention Between Metal vs. Porphyrin-
Based Oxidation Reactions - Spectrosc0pic Methods ............................... 18
1. Optical Absorption Spectroscopy ............................................................... 18
2. ESR Method ................................................................................................... 23
3. Magnetic Susceptibility Measurement ..................................................... 26
4. Electrochemical Regularities ...................................................................... 28
Other Physicochemical Methods .................................................................... 29
5. Proton NMR of Metalloporphyrin
n-cation Radicals ......................................................................................... 29
6. IR of Metalloporphyrin n-Cation Radical ................................................ 30
7. Resonance Raman ........................................................................................ 33
An Overview of this Thesis ............................................................................ 35
CHAPTER H - PREPARATION METHODS AND SPECTROSCOPIC
PROPERTIES OF OXIDIZED COBALT PORPHYRIN S
1. Preparative Methods .................................................................................... 44
Instrumentation ............................................................................................ 47
2. Electronic Absorption Spectra .................................................................... 47
3. ESR And Magnetic Susceptibility .............................................................. 55
Infrared Diagnostic Band of the Cation Radicals ................................... 58

iv

Page

RR Vibrational Analysis of CoHOEP and its
Oxidized Products-Spectral Characteristics of
the Cation Radicals in the High-Frequency Region ............................ 60

CHAPTER III - IRON PROPHYRIN TI-CATION RADICALS:
SOLUTION RESONANCE RAMAN SPECTRA OF

OEP+°Fe(X)(X')
Experimental Section ........................................................................................ 75
Materials and Methods ..................................................................................... 75
Chemical Ox1dations ....................... 76
Results and Discussion ..................................................................................... 76
High Frequency RR Scattering from Iron
Porphyrin-IX n-cation Radical ....................................................................... 83

CHAPTER IV - RESONANCE RAMAN SPECTRA OF THE

I'I-CATION RADICALS OF COPPER, COBALT,
AND NICKEL METHYLOCTAETHYLCHLORINS:
VIBRATIONAL CHARACTERISTICS OF

CHLOROPHYLL MODELS
Materials and Methods ..................................................................................... 92
Results and Discussion ..................................................................................... 92

Supplementary Material—Preparation of trans-
octaethylchlorin—dzmfi),-d2(y,5) and -d4 as well
as octaethylporphyrin—dz using acid-catalyzed

exchange reaction ........................................................................................... 105
Experimental ..................................................................................................... 1 07
trans-octaethylchlorin-d2(y,5) ..................................................... 107
trans-06218thylchlorin-d4 ............................................................................... 110
trans-octaethylchlorin-d2(a,B) ...................................................................... 1 10
octaethylporphyrin-dz .................................................................................... 110
Conclusion ........................................................................................................ 11 1

CHAPTER V - NOVEL B-PYRROLIC SUBSTITUTION REACTIONS OF
TETRAPERFLUOROPHENYL PORPHYRIN

General Characteristics
a. Reduced Affinity to Metal Ions ................................................................ 115
b. Resistance Toward Acid Demetallation ................................................. 115

Page

c. Reduced Proton Affinity ............................................................................ 115
d. Optical Absorption Properties .................................................................. 116
e. 'H—N MR ........................................................................................................ 118
f. Reversible Oxidation Reactions of MTFPP
Derivatives .................................................................................................. 119
Experimental ..................................................................................................... 120
Results ................................................................................................................ 121
Cu(II) and Ni(II) Adducts ................................................................ 125
A301) Adducts .................................................................................... 130
Zn (II) Adducts ................................................................................... 132
Discussion .......................................................................................................... 132

CHAPTER VI - METAL-AXIAL LIGAN D VIBRATIONS IN

METALLOPORPHYRINS, THEIR IT-CATION RADICALS
AND OTHER RELATED SYSTEMS

Experimental ..................................................................................................... 142
Results and Discussion ................................................................................... 142

vi

1-1

1-2

1-3

5-1

5-2

LIST OF TABLES

Page
Spin Densities in Porphyrin Cation Radical ......................................... 8
The Nature of Axial Ligand(s) in Some Hemoproteins ................... 14
Proton and Deuteron NMR Chemical Shifts for Copper and
Silver Porphyrins and Their One-Electron Oxidation
Products ....................................................................................................... 31
Proton NMR Spectra of Monomeric Iron Porphyrins ...................... 32
Proton NMR Resonances for Bis(imidazole)iron(III)
Porphyrin Radicals ................................................................................... 33
Resonance Raman Frequencies (cm‘l) for the Parent CoOEP
and its Oxidation Products ...................................................................... 67
RR Frequencies (cm'l) of Selected Iron OEP Species ........................ 86
Vibrational Assignments (1450-1700 cm‘l) for Ferric
Porphyrins and their n-Cation Radicals ............................................... 86
Resonance Raman Frequencies (cm‘l) and Optical
Absorption Maxima (nm) for Parent MOEP and MMeOEC
and their Corresponding n-Cation Radicals ...................................... 101
Estimated K(cm'1/A) and A(A) Values for the u and t)
Vibrations of the Parent MOEP, MMeOEC and their
It-Cation Radicals ................................................................................... 102
Absorption Spectra (nm) of Dicationic Salt of HZTFPP in
CHZCIZ ....................................................................................................... 116
Comparison of HzTFPP Absorption Maxima (nm) in
CH2C12 with Those of HzTPP and HZOEP ........................................ 117

vii

5-7

6-1

Page

Solvent-induced Absorption Maxima Shifts (nm) of TFPP
Adducts ..................................................................................................... 118

Changes in the Optical Absorption (nm) in CHzClz of the

AgII and CoII Complexes of TFPP Upon One-Electron
Metal-Centered Oxidation ..................................................................... 119

Absorption Maxima (nm) of TFPP and TPP Adducts in
CH2C12 ....................................................................................................... 129

Absorption maxima and FAB-MS Data on AgTFPP
Adducts ..................................................................................................... 130

Absorption Maxima (nm), FAB-MS m/z and 1H-NMR B-
proton Multiplicity Number (MN) of ZnTFPP Adducts ................ 134

Stretching Frequencies (cm'l) of (Fe-Cl) and (FeOFe)

Vibrations in Several Different Ferric Porphyrin (P)
Complexes ................................................................................................ 141

viii

LIST OF FIGURES
Page

1-1a Oxidation of hemoproteins porphyrin-iron
centers in biological systems. ............................................................... 2

1-1b Oxidation mode of photosynthetic chlorophyll a ............................. 2

1-2 Chemical structures of: (a) protoporphyrin IX and
its synthetic analogues; (b) meso-tetraphenyl-
porphyrin and (c) octaethylporphyrin. ............................................... 5

1-3 The atomic orbital (AO) structure of two HOMOs
of porphine. The A0 coefficients are proportional to
the size of the circles; solid lines indicate positive
values, dashed lines negative. The view is from the
positive 2 axis. The straight dashed lines indicate the

nodes of the am (It) orbital. ................................................................... 8

1-4 Shape and nodal characteristics of the four essential

porphyrin n-molecular orbitals ("Frontier" orbitals;
am and am are filled, eg are unoccupied). ...................................... 10

1-5 Typical n-electron energy level scheme of a closed-
shell metalloporphyrin: (A) one-electgron energies

of highest occupied and lowest unoccupied 1:-
molecular orbitals; (B) configuration energies for

the lowest n-excitations before (left) and after
(right) configuration interaction. (From Wang gt a)”
I. Am. Chem. Soc, 1984, m6, 4235.) ................................................... 20

1-6 Visible absorption spectrum of octaethylporphyrinato-
nickel(II), Ni(OEP), in CHzClz. (From Kitagawa gt a1_.,
Struct. Bonding (Berlin), 1987, _6_4, 75.) .............................................. 20

ix

1-8

1-9

1-10

2-3

Page

Oxidation of MgOEP in CH2C12. The solid line is the
absorption spectrum of Mg(II)OEP, the broken line that
of [Mg(II)OEP]+°ClO4'. (From Ref. 16a.) ........................................... 22

Oxidation of MgTPP in CHzClz. The solid line is the
absorption spectrum of Mg(II)TPP, the broken line that

of [Mg(II)TPP]+°ClO4'. (From Ref. 16a.) ........................................... 22

Upper spectra are those of [Co(III)OEP]7-+°2CIO4‘

(solid line) and [Co(III)OEP]2+°ZBr'(dotted line). The

lower spectra are those of the primary complexes of

horseradish peroxidase (solid line) and catalase (dotted

line). (From Ref. 16a.) ........................................................................... 24

RR spectra of (a) CuOEP; (b) CuOEP+'ClO4'. CHzClz
bands are marked with an *. CW laser power 20-35 mW. ........... 36

Visible absorption spectra of CoOEP (Soret = 391 nm)
and Com(MeOH)zOEPBr‘ (Soret = 411 nm) in dichloro-
methane. .................................................................................................. 49

Electronic absorption spectra of CoHOEP derivatives:

(a) CoIIOEP(---); (b) CoHOEP+-C104'(-—-);

(c) ComOEP+'2ClO4'(....), in CHzClz; (d) spectral changes

on addition of methanol to a methylene chloride solu-

tion of (b) producing Com(CH3OH)zOEPClO4‘, total

methanol content < 0.01%. .................................................................. 50

Electronic absorption spectra showing the conversion of'
CoHOEP+°CIO4' to Com(HzO)ZOEPCIO4' as the tempera-

ture is lowered. Solvent, "dry" CHzClz; path length ~ 2mm
(EPR tube). ............................................................................................... 52

Electronic absorption spectra of oxidation products of
CoOEP. (a) Com(MeOH)zOEPBr'(-); (b) ComOEPBr'(--);
(c) ComOEP+'ZBr'(---), the small feature at 401 nm is due to
1% H4OEP2+28r' contamination as discussed in the text.

Solvent, dry CHzClz, except for (a) which contains ~ 5%
methanol (MeOH). ................................................................................ 54

X

2-6

2-10

2-11

Page

250-MHZ proton NMR spectrum of ComOEPBr' in CDC13
at 25°C. Impurity peaks labeled X. ..................................................... 54

Spectral changes on addition of methanol to a dichloro-
methane solution of ComOEP+°2ClO4‘ in the visible

region producing Com(CH3OH)ZOEPClO4', total
methanol content ~ 2%. ....................................................................... 56

Infrared spectra (in KBr) of CoHOEP and its oxidized
products. The porphyrin cation radical diagnostic
band is labeled with an asterisk. ......................................................... 59

RR spectra of cobaltous octaethylporphyrin in CHzClz. ............... 61

RR spectra excited at 363.8 nm (~ 35 mW) of CoOEP and its
oxidation products. (a) CoOEP; (b) Com(MeOH)zOEPClO4';
(c) CoHOEP+-ClO4’; (d) ComOEP+-2CIO4‘;
(e) Com(MeOH)zOEPBr'; (f) ComOEPBr';

(g) ComOEP+°ZBr'. Solvent, dry CH2C12 except (b) and

(e) which contain ~ 5% methanol. Solvent bands are
marked with an * .................................................................................. 63

Electronic absorption spectra of oxidation products of
CoOEP.(a) Com(MeOH)2OEPBr'(-); (b) ComOEPBr' (--);

(c) ComOEP+°ZBr' (m), the small feature at 401 nm is due to
1% H40EP2+ZBr’ contamination as discussed in the text.

Solvent, dry CH2C12, except for (a) which contains ~5%
methanol (MeOH). ................................................................................ 64

RR spectra of cobaltic octaethylporphyrin 1t cation radicals
in CHzClz. ............................................................................................... 66

Absorption spectrum of FeIHOEP+-(Cl')(SbC16') in
CH2C12 ...................................................................................................... 77

Near-UV RR spectra of OEP+°Fem(X)(X') complexes. The

laser power was 35-40 mW. The solvent (CHzClz) band at
1423 cm"1 is labeled with an asterisk (*) ............................................ 79

xi

3-3

3-5

4-1

4-2

5-1

Page

RR spectra of ferric chloride porphyrins in CHzClz ....................... 81

RR spectra of ferric chloride porphyrin 1t cation radicals
in CH2C12- ............................................................................................... 84

RR spectra of model compounds in CHzClz. Conditions:

25-30 mW incident on the sample in a spinning cell at
25°C. .......................................................................................................... 85

Chemical structures of copper(II) derivatives: (a) trans-
octaethylchlroin (t-OEC); (b) methyloctaethylchlorin
(MeOEC). .................................................................................................. 91

Optical absorption spectra of CuMeOEC(-) and its one-
electron oxidation product, CuMeOEC+°CIO4' (—-) in
CH2C12 ...................................................................................................... 93

N ear-uv RR spectra of t-CuOEC (a), Cu (b), Co (c), Ni
(d) complexes of MeOEC. The laser power was 20-30 mw.

The solvent (CHzClz) band at 1423 cm'1 is labeled with an
asterisk (*). ............................................................................................... 95

RR spectra of (a) t-CuOEC+', (b) CuMeOEC+°, (c) CoMeOEC+°,

(d) NiMeOEC+°. The laser power was 20-30 mw. The

solvent (CHzClz) band at 1423 cm"1 is labeled with an

asterisk (t). ............................................................................................... 96

Chemical Structures. ........................................................................... 106

250-MHZ proton N MR spectra of t-OEC and its
deuterium labeled derivatives in CDC13. S indicates

residual undeuteriated solvent and X represents
other impurities. .................................................................................. 108

The electron impact mass spectra (El-MS) of trans-octaethyl-
chlorin and its deutero derivatives at 70 ev. .......................................

Absorption spectra of tetrakis(perfluorophenyl)
porphyrin complexes taken in CHZClz at room

temperature: (A) free base; (B) Zn complex, (C) Cu
complex; (D) Pd complex. (From Ref. 7.) ........................................ 109

xii

5-3

5-4

55a

5-5b

5-6a

5-6b

Page

Solvent-dependent absorption spectra of zinc tetrakis
(perfluoro-phenyl)porphyrin in visible region: (A) dry
toluene; (B) toluene shaken with HZO;(C) 75% toluene
and 25% CH3CN; (D) 75% toluene and 25% ethanol;

(E) 75% toluene and 25% pyridine; (F) 75% toluene and
25% triethylamine. (From Ref. 7.) ...................................................... 123

Electronic absorption spectra of CuTFPP(--) and Cu(2-B)
in CH2C12 ............................................................................................... 124

Absorption spectrum of H Ag(4-B) in CHzClz. ....................................

Expanded B-pyrrolic region of 1H-NMR spectrum of
Zn(2-B), fraction 2, in CDC13 .............................................................. 127

Expanded B-pyrrolic region of 1H-NMR spectrum of
Zn(2-B), fraction 4, in CDC13 .............................................................. 128

Electron absorption spectrum of H2TFPP/ AgN O3 / HOAC
reaction products; t<1 h. ”The 593 nm band is due to the

B-pyrrolic adducts. "Gouterman reports AgTFPP
Am Ex at 416, 536 and 570 nm in ¢CH3 ............................................ 133

Partial FAB mass spectrum of AgTFPP prepared by
AgNO3/HOAC method in < 1 hour. The m/z at 1081

(M+,AgTPP) is not shown .................................................................. 134

UV-vis spectra of CoOEPX in CHzClz. ............................................ 143

Absorption spectrum of ComOEP+-2X' (where X =
FeCl4' or C1.) in CHzClz. .................................................................... 145

UV-vis spectra of FeOEPX in CH2C12 .............................................. 146

Absorption spectrum of FemOEP+°(Cl')(SbCl6‘) in
CHzClz .................................................................................................... 147

RR spectra of CoOEPX in CHzClz excited at 363.8 nm.

The solvent bands at 1422, 703 and 283 cm'1 are labeled
with an asterisk. ................................................................................... 148

xiii

6-6

6-8

Page
RR spectra of FeOEPX in CHzClz excited at 363.8 nmm .............. 150

(a) R spectra of S4/56FeOEPF in CHzClz excited at

363.8 nm; b) R spectra of 54/ 56FeOEPCl in fH excited at
413.1 nm. ................................................................................................ 152

RR spectra of FeOEPX in benzene excited at 406.7 nm.
The solvent bands are marked with an asterisk. .......................... 153

RR spectra of ComOEP+°2X' in CHzClz excited at
363.8 nm: (a) X' = FeCl4' ~ C1-; (b) X' = Br‘. .................................... 155

xiv

CHAPTER I

INTRODUCTION

The diversified functions of hemoproteins in biological systems
include roles such as the transport of electrons (e.g., cytochrome b5), the
transport of oxygen (e.g., hemoglobin) and the catalysis of redox reactions (e.g.,
horseradish peroxidase and cytochrome P—450).1'3 These proteins contain an
iron (III) protoporphyrin-IX as their prosthetic group. Other related systems,
containing a saturated porphyrin ring macrocycle, mediate the rapid flow of
electrons in photosynthetic reaction centers (e.g., chlorophyll a, a magnesium
chlorin) and catalyze the reduction of nitrite and sulfite (e.g., siroheme, an
iron isobacteriochlorin).4r'5 Intuitively, it may be recognized that the
widespread use of heme family prosthetic group arises from the presence of a
macrocyclic unit, which contains an extensive delocalized n-system (e.g., 18 pi
electrons) as well as an iron atom coordinated in the central cavity and, that
both the iron and the rt-system are capable of rich electron-transfer chemistry
and reversible redox reactions, Figure 1.6 The system is tuned for a particular
function by means of specific macrocycle peripheral substituent(s), metal axial
ligand(s), and the structural influence of the surrounding protein pocket
which imposes both conformational and axial coordination restrictions on
the encapsulated heme center. For example, horseradish peroxidase (HRP)

and catalase (CAT) are two distinct enzymes that react with hydrogen

9/0
HEMOGLOBIN ,. o, z,
mocLoem Ft ,_..—-* Fe
crrocnnoues a.” _. h:-
.-
I
HZO? "2° 0 "2°: 0:
CATALASE a? >_ Z 2; .p" CAT ‘.3.
pEROXlOASE HRP
2 RH 2 R'
RH 90H
"0:5 C02” r r
HE ME CY TOCHnOME P «so Fe c.
02' 2... 2H. "20
crrocaaoue Z“? 2“;-

Oz ,4¢-,4H‘ ZHZO

Figure 1a. Oxidation of hemoproteins porphyrin-iron centers in biological systems.

 

Figure 1b. Oxidation mode of photosynthetic chlorophyll a.

peroxide to produce transient oxidized intermediates referred to as HRP I and
CAT 1, respectively, which possess two additional oxidizing equivalents
relative to the resting ferric state.7 Compound I in both species has been
assigned to a ferryl porphyrin rt-cation radical in which one oxidizing
equivalent is stored in the form of a tetravalent iron stabilized by oxygen
(FeIV=O) and the second equivalent resides in a porphyrin it-cation. In the
HRP case, the fifth proximal ligand to iron is a neutral histidyl imidazole,
whereas the iron atom in CAT bears a tyrosine phenolate. This distinction in
the nature of the heme-iron axial ligand is presumably the principal factor in
the functional differentiation of these hemoproteins.8 The macrocyclic 1t-
cation radical species has also been observed in the chlorophyll
photochemical reaction for the primary products generated by the
photooxidation of the chlorins in the photosystem I and II and, possibly in the
cytochrome P-450 reaction cycle.9 In order to account for the contribution that
each structural parameter makes to the overall reactivity, synthetic analogs of
hemoproteins are invaluable in this regard because they provide the chemist
with an opportunity to introduce a composite of several different structural
parameters in a stepwise fashion and, consequently, clarify the details of
biological redox mechanisms that involve hemoproteins iron-porphyrin
centers. Additionally, the protein-free heme model complexes are more
frequently responsive to detailed scrutiny than the hemoproteins themselves.

While use of specific peripheral substitution pattern of protoporphyrin
is essential for the elucidation of biological function, its chemical reactivity
and asymmetric distribution make this porphyrin a poor substrate for in vitro
model studies. Instead, we utilize two synthetic porphyrins,

octaethylporphyrin (OEP) and meso-tetraphenylporphyrin (TPP), which,

owing to their high solubility, chemical stability and relative ease of
preparation, have proved to be exceptionally useful for model studies. The
OEP complexes, however, will be the main focus of this study owing to their
biologically-relevant substitution pattern, analogous to that of
protoporphyrin species, Figure 2. Focusing on these structurally-related
porphyrins will demonstrate systematically how the model compound
reactivity may be tuned to obtain a desired potential (E0), a desired number(s)
of electrons transferred in the reaction sequence,or a specific ultimate product
(e.g., change in the valance of the central metal or generation of a n-cation

radical).

Research Objectives

This study aims to utilize synthetic porphyrins and their derivatives to
establish vibrational, electronic and structural characteristics of the oxidized
states of hemoproteins iron-porphyrin centers. The emphasis will primarily
be placed on the porphyrin-based n-cation radical state which serves as one of
the two oxidizing equivalents in the reaction cycle of peroxidases, catalases
and photosynthetic chlorophylls. At the time that the present work began,
the redox behavior of metalloporphyrins had been widely investigated and
there were already numerous reports available on the visible absorption,
ESR, magnetic susceptibility and NMR properties of these radicals;10‘13
however, their vibrational characteristics had not been studied in any detail.
This may partially result from the limited stability of these radicals, their

preparation under conditions (e.g., solvent, co-solvent and concentration)

 

COZH COZH
(a)

 

(b) (c)

Figure 2. Chemical structures of: (a) protoporphyrin D( and its synthetic analogues;
(b) meso-tetraphenyiporphyrin and (c) octaethylporphyrin.

that may not be suitable for vibrational studies, and the time required to scan
a spectrum when a conventional spectrometer is used.

Resonance Raman (RR) spectroscopy has been utilized as a powerful
probe, in the last few decades, to study hemoproteins structures, functions,
and mechanisms. A major advantage of R is its selectivity for individual
chromophoric species in a multicomponent hemoprotein system. Since the
interpretation of RR vibrational frequencies relies heavily upon previous
studies of model compounds that were designed to mimic the structure of
molecular species thought to exist in the more complicated protein sample,
this prompted us to investigate vibrational characteristics of synthetic
metalloporphyrin n-cation radicals. A systematic study of these radicals will
demonstrate the sensitivity of RR frequencies to the vibrational, electronic
and structural changes accompanied by removal of an electron from the
porphyrin-based it-system. This will, in turn, aid detection and assignment of
this particular mode of oxidation in the catalytic cycles of hemoproteins
currently under investigation. The RR data obtained in this study are
intimately supported by other spectroscopic methods, in particular, optical
absorption, ESR, magnetic susceptibility and IR data.

It is the objective of this chapter to present a general background of the
structural parameters involved and the methods commonly used in the

study of metalloporphyrin redox reactions.

Porphyrin n-Cation Radicals
The striking feature of the porphyrin ring is its ability to undergo facile

oxidation under chemical or electrochemical conditions to form a 1t—cation

radical. Probably the first direct evidence for the existence of porphyrin 1t-
cation radicals was Commoner's observation of a rapidly decaying, sharp ESR
signal at g = 2.00 in photosynthetic material.” Another early report on
signals that were similar but of high stability was made for solutions of TPP.15
Easy access to stable cation radical as well as their detailed analysis, was
achieved when ESR spectra, absorption spectra and redox potentials were
quantitatively related to the oxidative formation of these radicals and
crystalline material was obtained.16

Gouterman17 has shown that in a metalloporphyrin (MP), the highest
filled orbitals, am and am, are nearly degenerate, Figure 3. Removal of an
electron from these orbital produces a n-cation radical and the remaining
unpaired electron occupies either an am or an orbital with zAlu or 2A2u
ground state (D411 symmetry), respectively. Based on recent calculation by
Zerner,18 the two ground states differ in energy by about 4 kcal/ mole; thus,
small perturbation such as changes in peripheral groups, axial ligand(s) or the
central metal could determine the symmetry of the ground state. Moreover,
MO calculation as well as ESR data showed that in the 2A211 state, high spin
density appears at both the meso-carbon and nitrogen atoms, while in the
2A1u state, spin density is primarily localized on the a carbons, Table 1.19 For
example, the ESR evidence confirms that, indeed, the n-cation radical of
MgOEP occupies a 2A1u and that of MgTPP a 2A2u state.20

As a rule, the am orbital is found to be higher than the a1u orbital.

Taking into account the fact that am orbital has its maximum electron density

on the nitrogen and meso-carbon atoms, we can assume that addition of

meso-substituents (e.g., phenyl groups in TPP) will lead to an increase in the

 

Hm-l ”32“ int"

Figure 3. The atomic orbital (A0) structure of two HOMOs of porphine. The A0
coefficients are proportional to the size of the circles; solid lines indicate
positive values, dashed lines negative. The view is from the positive 2
axis. The straight dashed lines indicate the nodes of the am (it) orbital.

Table 1.

Spin Odie! in Porphyrin
Cation Radicals

 

 

Morn (calculated) (calculated)

 

C-l - 0.0094 0.098 l

C -2 0.0l 34 0.0262

C-5 0. l932 0.00l 2
N 0.049 0.000

 

(From Ref. 19.)

energy of this orbital to an extent that it becomes higher than the energy of

the am orbital. In this case, the electron abstraction will take place from this
orbital to form a 2A2u n-cation radical. Indeed, almost all metallotetra-
phenylporphyrin cation radical display a 2A2u ground state. In the OEP case,
the introduction of alkyl substituents in the pyrrole rings probably results in
the alu orbital being higher than the am level. The electron density will

therefore be removed from the orbital with am symmetry to form a zAlu

cation radical. However, several investigations have demonstrated that the
OEP-type system is more responsive to the change in the ground-state
symmetry as a result of perturbation caused by metal or axial ligand, than the

corresponding TPP complexes.21

Metal-Porphyrin Interactions
The role of the centrally coordinated iron in hemoprotein catalysis is to
hold the reacting species in conformations favorable for reaction and act as a
source of, or sink for, electrons. Calculations on model compounds have
shown that metal interacts with the porphyrin ring in two ways, which are
referred to as inductive and conjugative effects.”-
1. Inductive Effect: This mode of interaction results from the change of
electron density at the nitrogen atoms as the result of substituting

different metals in the central cavity. This perturbation affects the

energies of the doubly degenerate LUMO eg(1t*) and HOMO a2u(1t) but
not alum) orbital because the latter has nodes through the central
nitrogen atoms, Figure 4. As the central metal becomes less

electronegative, the sigma electron shell shifts from the metal towards

the ring which raises the eg and am orbitals by inductive effect.

10

 

 

 

Figure 4. Shape and nodal characteristics of the four essential porphyrin 1:-
molecular orbitals ("Frontier” orbitals; a2“ and am are filled, e8 are

unoccupied).

11

2. Conjugative Effect: This effect results from the interaction of metal P,t
orbitals with those of porphyrin it-electron system (e.g., eg and am). For
example, as the central metal becomes less electronegative, that part of
the pi electron density located in the metal 4P2 orbital can move back

into the ring causing an increase in the energy of the an orbital at the

expense of a decrease in conjugation between metal and porphyrin pi

orbitals.

Site-Selective Oxidation: Metal- vs. Porphyrin-centered Oxidation Induction

Metalloporphyrin redox reactions may be classified into three
categories:

1. Reversible changes in the formal oxidation number of the metal

("inorganic redox reactions"),

2. Reversible changes of the oxidation state of the porphyrin ("organic

reversible redox reactions") and

3. Both metal and the porphyrin ligands undergo reversible redox

reactions.

As was previously noted, several structural parameters contribute to
the selective oxidation reactions of the metal and porphyrin centers. These
are:

1. the nature of the central metal,

2. macrocycle peripheral substituent(s),

3. metal axial ligand(s),

4. solvent (or medium) and

5. temperature.

The role of these parameters will be discussed individually in the following

section and examples will be given.

12

Effect of the Central Metal: The redox chemistry of metalloporphyrins is
dominated by strong o-electron donation from the nitrogens to the metal
ion. Metals of low electronegativity and low oxidation number activate
the porphyrin ring with respect to oxidation and deactivate it with
respect to reduction. The opposite is true for metals of high
electronegativity and/ or oxidation number. Such reversible reactions of
the macrocyclic ring are of importance in biological systems containing
magnesium metal; namely, photosynthetic chlorophyll a. On the other
hand, electron-rich transition metal ions (e.g., iron) with formally high
oxidation numbers are stable in the porphyrin cavity and tend to transfer
some electrons to the axial ligand. This effect is of biological importance
in hemoproteins, which function in electron transfers and oxygen

activation.

Porphyrin Peripheral Substituent(s): The one-electron oxidation
products of the MP's are highly reactive species having redox potentials
between those of 12 and Brz (~ 0.5 - 1.2 volts). Experimentally, it is found
that oxidative titration in organic solvents with a stoichiometric amount
of oxidant are only possible for complexes with midpoint potential below
0.3 volts (e.g., magnesium octaethylchlorin, MgOEC; ZnOEC and BChl a)
and iodine as an oxidant. In all other cases, an excess of oxidant is
needed to complete the oxidation of MP probably because the organic
solvents, or reducing impurities contained in them consume oxidant

above 0.5 volts. A range of oxidants may be used to produce n—cation
radicals including 12, Brz, CuBrz, NBS, DDQ, Fe(ClO4)3, FeC13, pheno+-
SbCl6', ¢3N+X’, CeIV salt, XeF223. In special cases, stable cation radicals

13

of porphyrin itself may be used to induce oxidation in another porphyrin
system such as ZnTPP+-ClO4'(81 /2 = 0.77 volts vs. SCE) and

(FeTPP)zO+°ClO4'(81/2 = 0.84 volts vs. SCE).24

Recent findings concerning the role of the propionate and vinyl
groups of iron (III) protoporphyrin-IX in cytochrome b5 highlights the
significance of macrocycle peripheral substitution pattern. This study,
reported by Reid e_t a_l,25 indicated that heme proteins modulate the
reduction potentials of their heme redox centers by constraining the
position of the vinyl groups with respect to the plane of the heme and
thereby affecting the electron-withdrawing ability of the vinyls. By
forcing the vinyl substituents into the plane of the heme prosthetic
group, their electron-withdrawing ability is enhanced, and the reduction
potential of the heme center is increased. In another study, they also
suggested distinctly different roles for heme propionate groups, one
partially responsible for determining the reduction potential of the iron
center, and the other group is involved in recognition of and interaction
with heme proteins that are physiological redox partners of cytochrome
b5. As demonstrated by model studies, in general, the presence of
electron-donating or electron-withdrawing substituents modify the
electron density at the central metal and lead to shifts in redox potentials,
the magnitude of which are dependent on the substituents and their
orientation with respect to the macrocycle ring plane. For example,
metallochlorins are oxidized by as much as 0.3 volts lower than
metalloporphyrins, and bacteriochlorins are again 0.3 volts lower. This

trend can be rationalized by noting that in the B-hydrogenated

l4

porphyrins, one or two of the electron-withdrawing pyrrole units, which
can be formulated as pyrrole anions (pk~16), are replaced by pyrroline
units, which function as aza bridges (pk~2) in the inner conjugation
path. Therefore the ligand becomes less basic and the oxidation potential
of the central metal increases; therefore, it becomes easier to remove an

electron from the porphyrin ligand itself.26

Metal Axial Ligand(s): Metalloporphyrins can bind one or two ligands at
the axial coordination sites and these ligands can potentially dictate the
chemistry of the heme group. The heme in the electron carrier
hemoproteins is exceptional in that it has two strong axial ligands and
generally does not bind molecular oxygen or peroxides. The heme in all
other hemoproteins has one accessible coordination site which allows it

to react with peroxides and other ligands (Table 2).

Table 2. The Nature of Axial Ligand(s) in Some Hemoproteins

 

l
‘ Hemoprotein Axial Ligand(s)
Cytochrome P-450 Cystein thiolate (1)

HRP Histidine Imidazole (1)

CAT Tyrosine Phenolate (1)
Cytochrome c peroxidase Histidine Imidazole (1)
Cytochrome b5 Histidine Imidazole (2)
Cytochrome c Histidine Imidazole (1)

Methionine Methylthioether(1)

 

 

 

The number in parenthesis indicates the number of axial ligand(s)

coordinated to the heme-iron center.

15

The nature of the axial ligand can trigger metal vs. porphyrin-based
oxidation, influence the ground state symmetry of the porphyrin radical
(2A2u or 2Am), alter the spin state of the central metal and disturb the
conformation of the macrocycle.

For example, the site of oxidation of ruthenium(II) porphyrins
(metal or porphyrin) is dependent on the nature of the axial ligand.
When pyridine occupies the extraplanar sites, oxidation occurs at the
ruthenium ion. If CO is one of the extraplanar ligand, the site of
oxidation is at the porphyrin ring. This change has been attributed to
stronger ruthenium back-bonding to CO compared to pyridine, which
stabilizes metal d1; orbitals relative to the porphyrin 11: levels; the latter
then becomes the valance MOs of the system.27a

It has also been shown that in iron(III) tetramesitylporphyrin,
TMPFeIH(L), strongly basic ligands such as L = o‘,OH',OCH3',F' and = O
favor the formation of iron (IV) species, whereas weakly basic anions
such as L = ClO4‘, Cl' and imidazole cause porphyrin cation radical
formation.27b

As discussed earlier, the species COIHOEP+°2X‘ can be made to

occupy either of the two ground states (2A1u vs 2A2“) as a function of its
axial ligands. When the dibrornide (2A1u state) is treated with AgClO4,
the bromide ligands are removed and the resultant diperchlorate salt
now shows characteristics of the 2A2u state.16a

The variation of the axial ligand can result in a change in
Aspin state, particularly of iron porphyrins, and concomitant structural

changes.28 High-spin hemes (S = 5/ 2) are five-coordinated and exist in

16

square-pyramidal conformation. The iron atom is displaced from the
porphyrin plane toward the axial ligand (e.g., X‘ = Cl',N3') which makes
the binding of further ligands unlikely. On the other hand, the
formation of two strong axial bonds (e.g., bis imidazole derivative)
involves an in-plane position of iron and a low-spin state (S = 1/2).
Thus, the obligatory structural changes that accompany the low-spin to
high-spin transition is an expansion of the porphyrin core size (e.g.,
PFem(Im)2+ —> PFem(DMSO)2+). High-spin Fe(II) and Fe(IH) porphyrins
are both too large to fit into the central cavity of the porphyrin ring and
they are forced out of the porphyrin ring by ~ 0.4 and 0.5 A, respectively.
These out-of-plane structures exhibit significant doming and ruffling of

the porphyrin ring.29

Solvent (or Medium): Protic solvents (e.g., CH3OH) slowly destroy the

cation radical and may only be used as co-solvents in oxidation reactions,
when the electron transfer is very rapid. Although, the stabilizing effect

of nucleophilic solvents on the oxidation of certain metalloporphyrins

(e.g., MgOEP) favors the use of solvent mixture such as CHC13-

CH30H(4:1);10 however, the electrophilic nature of the porphyrin cation

radicals may cause nucleophilic addition of the solvent to the bridged
meso carbon of the cation radical and the subsequent formation of an
isoporphyrin complex.30 Thus, care must be taken to avoid occurrence
of this side reaction.

Solvent effects on the porphyrin redox potentials are small.

However, solvent effects on metal redox potentials can be extraordinarily

17

large. For example, the CoIH/CoII redox couple involves low-spin d6/ d7
configurations. Strongly binding axial ligands (solvent molecules)
destabilize the dzz electron (in d7) and favor oxidation to low-spin d6
species. Thus, in this case, the potential shifts negatively with increasing
donor strength of the solvent (e.g. DMA = DMF< DMSO < pyridine).31
Solvent may also play a role in the selective formation of metal-
or porphyrin- centered oxidation product. The first oxidation of CoOEP
in dry CHzClz, for example, involves formation of a Co(II) porphyrin
cation radical while the second and third oxidations involve abstraction
of electrons first from the Co(II) center and then from the porphyrin 1t-
system. This ordering in the sites of oxidation is reversed in
coordinating solvents or in the presence of nucleophiles. In the latter
solutions, the first oxidation of CoOEP invariably involves the

Co(II)/Co(III) transition.32

Temperature. Oxidation of NiTPP at room temperature results in the
abstraction of an electron from a porphyrin-centered 1t orbital to form a
n-cation radical. This species, NiTPP+; displays an exceptional thermal
behavior. As the temperature is lowered, an internal electron transfer
occurs, with the hole moving from the periphery to the metal to
generate a N i(III) species.3

Although the structure of protein determines the heme-iron spin
equilibrium in hemoproteins through control of axial 1igation,34 a
thermal spin-state equilibrium can be observed in a model heme system
whose axial ligation is characteristic of these proteins.35arb Several

studies have indicated that the nature of the axial ligand is an important

18

factor in determining the high-spin or low-spin character of the iron
center. Weak field ligands such as F' form high-spin, six-coordinate iron
(III) complexes; strong field ligand such as CN’ and imidazole form low-
spin hexacoordinate iron (III) complexes, and ligands such as N 3‘ having
intermediate field strength form complexes that exhibit temperature-
dependent, spin-state equilibrium.36 The position of the equilibrium at
a given temperature is sensitive to the immediate environment of the
iron(III) porphyrin cation. For example, OEPFeHI(py)2+ CIO4" show a
magnetic moment at 77°I< corresponding to an almost pure low-spin
state (S = 1 / 2) and a moment at room temperature appropriate for 1:1

mixture of the low-spin and high-spin (S = 5/ 2) states.3513

Differentiation Between Metal vs. Porphyrin-based Oxidation Reactions -
Spectroscopic Methods

1. Optical Absorption Spectroscopy: The oxidation reactions of metallopor-
phyrins usually can be followed visually: a neutral metalloporphyrin
gives a reddish color solution; oxidation most often changes this color to
dark brown or green if the reaction is porphyrin-centered, whereas the
red color maintains if the reaction proceeds at the central metal“), 32

Since MPs and their oxidized products display characteristic
electronic absorption spectra, the following section will briefly address
the optical properties of these species.

A typical absorption spectrum of the metal complex of the

biological-type (spyrrole-substituted) porphyrin (e.g., OEP) features two
visible bands (at or 00,0 and B or Q03 bands) of moderate intensity (8 z

19

102-103 M'lCm‘l) in the 500 - 600 nm region and a very strong (8 z 104-
105 M'lCm'l) band, called the Soret (also 8 or B) band near 400 nm,
Figure 6. The visible and near-uv spectra of MPs can be interpreted
within the context of the four-orbital model proposed by Gouterman and
coworkers.37 The electronic transitions that give rise to the characteristic

spectra of MPs arise by excitation from the two highest filled orbitals
(HOMO's), the nearly-degenerate a1u(1t)and a2u(1t) levels, into the

doubly-degenerate lowest unfilled orbitals (LUMO's), eg(1t*) levels, (D4h
symmetry). For the porphyrin ring, the lowest singlet excited
configurations, 1(a2u,eg) and 1(a1u,eg), are of the same symmetry (Eu)
and of similar energy, Figure 5. These transitions have strong electronic
interaction between them and are mixed by configuration interaction to
yield the relatively weak visible Q0,0 band in which the transition
dipoles nearly cancel, and the intense near-uv Soret band, in which the

transition dipoles of the two configurations add. The closer the

degeneracy of the configurations '(a2u,eg) and '(alu, eg), the weaker is the
Q0,0° In some instances (e.g., CoTPP), the Q0,0 band essentially
disappears. In addition, the Q0,l vibronic overtone is also active and
appears as an additional peak on the high-energy side of 00,0 transition.
The Q0,1 band arises from vibronic mixing of the Q0,0 state with the B
state and is constant among porphyrins.38 The assignment of this band
is based on the constancy of its energy separation from the Q0,0 band and
its nearly constant intensity.

Comparison of the optical spectra of a number of n-cation

radicals16a reveals that they fall in two categories typified by either

20

 

 

 

A 3
Po“ Alter
Orbitoi Energies Configurations C! a
”"1:—
I

(o.‘. s: i—‘

. .. I T“

cum

I I

Ground Slots

 

 

‘.to E

 

 

 

 

Figure 5. Typical zit-electron energy level scheme of a closed-shell
metalloporphyrin: (A) one-electgron energies of highest occupied and
lowest unoccupied n-molecular orbitals; (B) configuration energies for the
lowest n-excitations before (left) and after (right) configuration
interaction. (From Wang gt 31., I. Am. Chem. Soc, 1984, 1% 4235.)

 

Ni(OEP)

 

ABSORPTION

Xl/2

 

 

l L I l L l
400 500 600
WAVELENGTH (nm)

Figure 6. Visible absorption spectrum of octaethylporphyrinatonickeKII). Ni(OEP),
in CHZClz. (From Kitagawa gt 3], Struct. Bonding (Berlin), 1987, fi, 75.)

 

21

MgOEP, Figure 7, and MgTPP it-cation radicals, Figure 8. The MgOEP
radical has a visible spectrum that is characterized by a major absorption
peak near 700 nm with a high-energy shoulder, while the MgTPP radical
exhibits a broad, near featureless absorption in the region 500-700 nm.
These differences have been ascribed to two close-lying ground states of
the radicals: 2A1u (class 1) for MgOEP+' and 2A2“ (class 2) for MgTPP+°
The preceding interpretation has been utilized for some time as
the optical criterion by which to distinguish between the two possible
ground states. Qualitatively perhaps, it is still useful, however
exceptions have been found and caution must be used in making any
definite assignments that are based on the optical characteristics. For

example, Godziela gt a_l. recently showed that CuOEP+°, previously taken

as an example of 2A2u ground state,16a exhibits NMR chemical shifts

characteristic of a 2A1u radical.39 A recent study by Zerner and

coworkers has provided a firm foundation for a quantitative
interpretation of MP n-cation radicals.18 This work demonstrates that
2A1“ cations lie lower in energy than the 2A2u cations by only 4
kcal/mole, consistent with the findings that both radicals are found
depending upon substituents and solvent. The visible region of the

spectrum of the 2A1u species is predicted to consist of three separate

bands decreasing in intensity with increasing energy, while that of the

2A2u species is calculated to consist of three allowed transitions of nearly

equal intensity, in accord with the experimental findings. The Soret

region of the 2Am ions is dominated by at least four allowed transitions

spread over 100 nm, while that of the 2A2u radicals contains at least

22

gore-0’

 

'0

 

 

 

 

0-.
coco-o..--------
0"
a
a

 

 

 

 

 

Figure 7. Oxidation of MgOEP in 01202. The solid line is the absorption spectrum
of Mg(II)OEP, the broken line that of [Mg(mosrlraoat (From Ref. 16a.)

 

 

 

 

 

 

 

 

 

 

me (out

Figure 8. Oxidation of MgTPP in CHzClz. The solid line is the absorption spectrum
of Mg(II)TPP, the broken line that of [Mg(II)TPPP'ClOf. (From Ref. 16a.)

23

three allowed bands, but the intensity in this region is dominated by only
one of them.

Thus, an absorption spectrum with two sharp bands in the visible
region and a narrow Soret band indicates a pyrrole-substituted MP with
the porphyrin ring in its zero oxidation state. If this spectrum is found
in an oxidation product from a MP, then oxidation has occurred at the
metal (e.g., AgHOEP -> AngEP).40. If, however, the electronic spec-
trum broadens considerably after oxidation, then the porphyrin ligand
might have reacted (e.g., CuHOEP —> CuIIOEP‘l").16a As a matter of fact,
similarity between the optical spectra of catalase and horseradish
peroxidase compound I with those of CoIIIOEP+° 2Br" and ComOEP+°

2ClO4', respectively, led to the notion that these complexes may be

characterized as porphyrin it-cation radicals and that, CAT I exists in the
2A1u ground state, whereas HRP I exists in the 2A2u state. It was also
speculated that differences in reactivity between the two enzymes are the
result of differences in the ground states, which may, in turn, be caused
by minor changes in the axial ligation, as demonstrated by the optical

characteristics of the two cobalt octaethylporphyrin cation radicals, Figure

9.16a

ESR Method. If only one unpaired electron is found in the product, then
its location is easily determined by examination of line width, g-values
and hyperfine structure in the ESR spectrum. Typically, the ESR
experiments yields two possible results:

1. An isotropic signal characteristic of an organic free radical around g =

2.00 and of line width less than 12 G (e.g., ZnTPP‘“-).19

24

 

 

ex IO"5

 

 

 

 

560 600 760

)\,nm

Figure 9. Upper spectra are those of, [Co(III)OEP]2*'2ClO4' (solid line) and

[Co(III)OEP]2*"28r'(dotted line). The lower spectra are those of the

primary complexes of horseradish peroxidase (solid line) and catalase
(dotted line). (From Ref. 16a.)

300 4b0

25

2. An anisotropic signal of line width > 50 G with various g values
typical of transition metal ions (e.g., CoIIITPP*’°).41

Copper porphyrin cation radicals, on the other hand, are more complex
owing to the presence of one odd electron on the central metal (3dx2-y2
orbital) and the other on the porphyrin ring. Therefore, their spin state
is either singlet or triplet depending on which is lower in energy. Since
no triplet ESR spectrum had been detected for some time for any copper
porphyrin radicals, it was speculated as being due to a singlet ground
state or to line broadening of triplet spectra.10 However, Konishi and
coworkers recently reported the first detection of a triplet spectrum of a
copper porphyrin cation radical produced in tetrachlorethane matrix at
77°I< by 8 radiolysisflr2

The ESR spectra obtained for MR“ can be classified into two types,
exemplified by 2A2u and 2A1u ground states. The ESR analysis of the

2A2u state reveals:

1. Large spin density at the meso carbons which can be transmitted to
the meso substituents (e.g., phenyl group).
2. Enough spin density at pyrrole nitrogens to cause nitrogen hyperfine
splitting (1-2G).
3. Zero spin density at B-pyrrole positions.
4. Enough spin density at the metal to cause metal hyperfine splitting
(e.g., azn = 1G).
5. Charge density transmission to the counterion (e.g.,
ZnTPP+-(Py)ClO4‘).
In the 2A1u state, on the other hand, small hyperfine splittings from
the meso protons are observed (e.g., an = 1.48G for MgOEP+°ClO4'). This

26

ground state also does not display any splitting due to the nitrogens, central
metal, or counterion and the electron density is mostly localized on the a

carbons.

3. Magnetic Susceptibility Measurement: When both the metal and the
porphyrin ring contain unpaired electrons, magnetic susceptibility
measurements by using a SQUID susceptometer in the solid state43 or
Evan's NMR shift method in solution44 help to locate the site of a
reaction. The theory of spin state in these MP4" is based on the
occupations and symmetries of the so-called magnetic orbitals, i.e., those
orbitals on the metal and the ligand that contain an unpaired electron.
MP4": complexes whose half-occupied d orbitals are strictly orthogonal to
the porphyrin n-radical orbital are expected to show ferromagnetic
coupling (like Hund's rule). Thus, the spin state of higher multiplicity
will be lower in energy. For example, in a strictly planar environment of
D4h symmetry as well as in the absence of any complication from
aggregation, CuTMP+°SbCl6’ features orthogonal copper dx2-y2 and
porphyrin am or a2u orbitals and shows an S = 1 (i.e., triplet), ueff =
2.9u3, ground state.45 On the other hand, complexes whose metal and
ligand magnetic orbitals are not strictly forbidden by symmetry to
overlap are expected to show antiferromagnetic coupling (or bond
formation). The spin state of lower multiplicity will be lower in energy.
This is exemplified by the CuTPP+' SbCl6' crystal which shows an S = 0
(i.e., singlet) ground state. This crystal exhibits a saddle-shaped
conformation in which nearly planar pyrrole rings tilt up and down and

the phenyl groups become nearly coplanar.4r6 Although this

27

conformation allows the appearance of tightly associated pairs of cations,
the face-to-face structure of the dimer lacks any recognizable specific
HOMO-LUMO interaction or the popular but unproven intermolecular
metal-nitrogen interaction. The large Cu-Cu separation (5.43 A) also
makes it unlikely for the trivial d-d coupling between cofacial dx2-y2
orbitals. For similar reason, the intermolecular d-n: coupling is also
considered unlikely. This leaves an intramolecular d-n: coupling within
each CuTPP+° molecule as the likely source of complete diamagnetism.
Thus, Reed and coworkers‘lr6 suggested that intramolecular metal-

porphyrin spin coupling occurs as a result of the lowering of the local
symmetry from D4h to Cs which destroys the orthogonality of magnetic

orbitals. Under CS symmetry, the dx2-y2 and the am orbital overlap

become symmetry allowed and the resultant spin pairing may be viewed
as copper porphyrin bond formation with the two electrons spin paired
in the bonding molecular orbital. In solution, however, the monomeric
CuTPP+° is planar and strict orthogonality of the magnetic orbitals gives
rise to the paramagnetic state.

A yet alternative mode of coupling is demonstrated by CuOEP+°
whose nearly planar conformation is prone to the formation of tightly-
spaced dimers (Cu-Cu separation of ~ 4A) at low temperatures or in solid
state and at high concentration in solution.45 This can reduce the
moment of the monomeric CuOEP+' triplet ground state (ueff = 2.9 113)
as a function of the extent of association into dimeric units or higher

order aggregates. The driving force for dimerization is probably best

28

viewed as a case of weak 1t—1t bond formation between two half-occupied
porphyrin HOMOs. Indeed, N iOEP+° is diamagnetic from presumed 1t-1t
coupling.46

Electrochemical Regularities: A systematic voltammetric investigation
of the redox potential of metal complexes of octaethylporphyrin
including all members of the first transition series has yielded some
useful electrochemical regularities that provide additional support for
the site of oxidation in metalloporphyrins.47

1. The differences in potential between the first oxidation and reduction

of the porphyrin ring are, with two exceptions (Mn, Mo), constant:
on_ - Bred. = 2.25 i 0.15V
2. The differences in potential between the removal of the first and
second electron are also constant:
E20,, - Elox. = 0.29 3: 0.05v

These empirical rules together with chemical and spectroscopic data may
be used to correlate various redox steps of a metalloporphyrin system
with metal vs. porphyrin-centered reactions. The more interesting of
these two electrochemical rules is certainly the first one: regardless of
any intermediate change in the oxidation states of the central metals, the
difference between the highest occupied and lowest unoccupied orbitals
is a constant 2.2 eV in the ground state. With stable divalent metals,
linear correlations between redox potentials and metal electronega-
tivities have been demonstrated.26 The fact that a rise in oxidation
potential is accompanied by a lowering of the reduction potential clearly

points to the inductive effects of the metals on the porphyrin n—system.

29

Voltammetric waves corresponding to potentials which are far
away from these patterns are expected to be associated with reactions of

the central metal ions.

Other Physicochemical Methods: In addition to the more conventional
methods of investigation described above, metalloporphyrin redox
reactions have recently been studied by other techniques including
NMR, IR, and resonance Raman (RR) spectroscopy. The utility of these
methods will be briefly discussed in the following sections; however, RR
vibrational analysis of macrocycle-based oxidized hemoprotein models
represents the main subject of this dissertation and will be covered in

detail in the following chapters.

Proton NMR of the Metalloporphyrin it—cation Radicals: The salient

features of 2A2u porphyrin it-cation radical are distribution of a
substantial amount of positive spin density at the meso carbons and the
pyrrole nitrogens, while the radical orbital in zAlu cations has a node at
these atoms (D4h symmetry).19 Since the isotopic shift of the meso
proton is proportional to the meso carbon spin density according to the
McConnell equation,8 the typical 2A2u radical is expected to induce a
large upfield contact shift for the meso proton whereas the zAlu radical
should exhibit a downfield bias.4*9 In the TPP complexes, the n-cation
radical character, described as an an radical type, is strongly implicated
by a large upfield and downfield spread of the phenyl proton signal,
Table 3. The most striking difference between high-spin and low-spin

iron(III) TPP radical is the fact that the signs of phenyl resonances are

30

reversed for the two species. Thus, phenyl ortho— and para-proton
signals are far downfield for the high-spin chloroiron(IH) species, Table 4;
but, shifts of the same magnitude, but upfield, are seen for the
bisirnidazole complex, Table 5. N 0 description is offered for this reversal
phenomenon; however, it is suggested that the phenyl shift pattern of
the high-spin iron(III) porphyrin radical is the exceptional case and that
the pattern for the low-spin iron(III) radical parallels the shift pattern of
several other recently identified MP4" (e. g., CuTPP+', Table 3). A shift of

the OEPFeCl methine protons upon oxidation, Table 4, is consistent with

the expected shift direction of known alu radicals, also demonstrated by

CuOEP+' C104“, Table 3.

Another interesting observation made by Morishima and co-
workers is concerned with the non-Curie law behavior of the

temperature dependence of meso-D resonance shift in ComOEP'l'QX'

(where X = Br',ClO4“).51 They interpreted this behavior in terms of
thermal equilibrium between 2A1u and 2A2u states. If we assume as
they suggested, that the am orbital possess slightly higher energy than
the an orbital in biological-type porphyrins, the thermal mixing of the

2A2u state increases with increasing temperature to afford more positive

spin density at the meso positions, and eventually inducing more

upfield isotropic shift.

IR of Metalloporphyrin n-cation Radicals. This method provides a
diagnostic tool for detecting porphyrin It—cation radical character, as was
first reported by Shimomura and Goff.52 For the TPP radical complexes,

an intense new band appears in the 1280 cm"1 region. Likewise, oxidized

31

Tab Lhasa-dMNHIMMI¢Ce~sd$~hmsuMOerMMnW

 

 

m est-st W pet-vi inn-I- M- has <3“:- ti-s 4“. «'1‘ {Hr
an)“. as us I?) 1.11 m a:
(movie I It I w (m
(OEPiAs' 1" ll 1.)! (“l
(oer-4.34.! -Is.I (on; i1.) um
iflPP-d.)AeiOO.' us (mi
"endgame: 9.0 (up 4.)! (m
(We‘ 1.“ (lat 1.10mi 1.31 (i0? is
IMJ-(OCHMI'MCE u ms) 3.“ my 1.” (i n- x
meat x 1.“ (m m ()9) at
(Tn-med u unset
Image-loo: I 3.4 I i711
l("'-J.)Cilmg' «.10 not us In) -I.oI (m
IOflicv x I L: (I213) LII Mi
(OH-axe 4.1 (20st ".3 (I!)
Itoeugcotoo: «.I (130) In (I21)
(ETIOKI' x ”J (tom u (Jon i.“ my
(filo-JuiCe’ 4.2 (203) it.) (“i u (m
l(ETlO-J,.)CulClO.' «sum it? user In (th

 

awmmmatuvmmmmmmmmmrs. Linewidthminfi
parentheses,areglveninHL'X'aitriesuidlatesigmlsnotdetectedduetolargelimwidthand/or
overlap with othersigmls. b(231303 solver". cCI-12Clzsolwent. dCD202 solvait. 'p-CH3 sight. ftn-

OCi-l3 siyul. 89-00-13 sight. (From Ref. 39.)

32

Table 4 . Inseam! Soecoadltmieirea Porphyr'us‘

 

 

 

 

canoe pyrrole «the meta para Cit.
ammo ”.4 -s i3.J. 6.33
In
(TMI’thOOJ “.i 17.6. -Iz.4 29.5
34.4
macaw-ac: 191 -s In. $.2i
ii.’
mum-warm.) 59.9 out 4.3 4.1
ti‘iOi‘ amt IIuI‘ (mt
rrru-ocngncmoom as 41.3 4.2 «a
coated Cit, Cit, mesa
(OEPiF-O‘ 0.1. 39.3 s.“ -54

(OEflFeOiOOJ‘ 10.5. 29.6 3. l6 -ll
8Except as noted oxidized species in CD202 solvent,
others in CDCl3 solvent. iron porphyrin - 10 mM. 26°C,
Me45i reference. bLine widths at half height.
scum solvent. dao-c; Itau-c. (From Ref. 49.)

Tale S.Protoa Null Reno-ans for listinirhaoieiitofllll)
Wrist lladieats‘

 

 

TPP- TPP-
proton «pocm‘ 17" (Lu-oat r 09‘ ETIO‘
pyrrole ~32.7 «on 40.4
o-oitenyi -36.J -Il.7
walnut 24.8 30.6 26.7
"heart -22.l
pOCH, i2.9 9.5
, I18
tin. CH, SIJ $0.4
tine CH, till

 

W2 solvent. 0.01 M iron porphyrin, 0.5 equivalent
of imidazole. referenced to (CH3)4Si; downfield shifts
are given positive sign. 968°C. C-3O‘C, 1.0 equivalent
imidazole present.d-51'C; ethyl-CH3 groups were
obscured by the solvent signal; the mesa proton signal
was not detected. (From Ref. 50.)

33

OEP radicals display a diagnostic band in the 1550 cm'1 region. Other
workers53 have criticized the use of this band as a diagnostic criterion,
and produce evidence suggesting that it could arise from the trapped
chlorocarbon solvent molecules in the crystalline cation radicals.
Hinman and coworkers have recently studied the cation radicals of
several metallotetraphenylporphyrins and concluded that the formation
of the n—cation radical results in two characteristic absorbance decreases
near 1485 and 1600 cm'l, and five characteristic absorbance increases near
1005, 1225, 1285, 1350, and 1415 crn-1.54 Investigation of the cation
radicals of OEP complexes was also recently carried out by Itoh and
coworkers.55 The tentative assignments of infrared bands of these
radicals indicate frequency shift patterns relative to the unoxidized
molecules that can be used for determining the ground electronic states

of the n-cation radicals.

Resonance Raman. The application of RR spectroscopy to biological
systems is an area of intense activity. RR spectroscopy is potentially able
to furnish information about a molecule and its environment under
experimental conditions which are not favorable to classical vibrational
spectroscopy (low concentration, complex media, aqueous solution).
Due to the phenomenon of resonance enhancement, specific parts of
complicated molecules (e.g., hemoproteins) may be probed without
interference from other parts of the molecule or from the solvent. The
utility of this technique as a structural probe of the chromophore in

hemoproteins has stimulated studies on protein-free metalloporphyrins,

yielding a great deal of information about structural, spectroscopic and
physical properties.563 Laser excitation into porphyrin Soret or visible
bands enhances the scattered intensity by 103 - 104 and provide selective
excitation of porphyrin vibrational modes. Empirical correlations
between the positions of Raman lines and metal oxidation, ligation and
spin state and the core size (center-to-nitrogen) distances exemplify some
of the information accessible by RR spectroscopy.56b Identification of

peripheral substituents and the dependence of the depolarization ratio

II
p = m of certain lines as a function of the wavelength of incident

radiation is interpreted as evidence for electronic symmetry reduction,
i.e., CuOEP(D4h) —) CuOEC(C2v).57

The dominant RR bands in the 1000 - 1700 cm'1 region correspond
to the in-plane stretching of C-C, C-N partial double bonds and the
bending of C-H bonds. At low frequencies, < 1000 cm'l, the RR spectra
are dominated by out-of-plane modes of planar MP, which correspond to
the bending of the in-plane bonds, as well as by modes that involve the
central metal. Experimentally, it has been found that an intense band at
~ 1360 cm'l, for pyrrole-substituted MP5, is sensitive to the metal
oxidation state. This band, which is commonly referred to as the

"oxidation-state marker, V4" corresponds to the breathing mode of C-N

bonds. Shifts to higher frequency in this line result from lower it-

electron density back-donation to the porphyrin eg(1t*) antibonding

orbitals (e.g., FeIII -) FeIV).58 Essentially, all the porphyrin skeletal

modes in the high-frequency region (1450 - 1700 cm'l) show a negative

35

linear correlation with core size for planar porphyrins.16b Roughly, a
decrease in frequency of 1 cm"1 in a core size line represent an increase in
core size of ~ 0.002A.56a Certain porphyrin modes in this region show

sensitivity to the metal spin-state. This is especially true of the band at ~

1580 cm'1 (v19, CaCm) which is relatively insensitive to the metal
oxidation state. Other spin-state marker bands appear at ~ 1490 (V3,
CaCm) and ~ 1630 cm"1 (v10, CaCm). For example, in high-spin iron(III)
porphyrin complex, electrons populate the antibonding orbitals (e.g., dxz-

y2) and the lengthened bonds are accompanied by the expansion of the

porphyrin core, followed by the decrease in the force constants of roughly
75% of the Raman active vibrational modes.58 Doming and ruffling of
the porphyrin ring also alters these frequencies, but their influence is less
important than the dominant core size effect.

Upon removal of an electron from the highest filled porphyrin 1t-

bonding orbitals, am or am, we find that the frequencies of the stretching
modes with predominantly Cbe character increase, whereas those with
CaCm and CaN character decrease in the RR spectra of the cation radicals

relative to the neutral metalloporphyrins, Figure 8. These structural

trends seem to be essentially insensitive to 2A2u vs. 2A1u radical

designation, at least for the cation radicals studied here, Figure 10.59

An Overview of this Thesis
For a complete and systematic investigation of redox, vibrational,
electronic and structural properties of metalloporphyrin n-cation
radicals, we begin by addressing the oxidation products of cobalt

porphyrins in Chapter 2. The versatility of cobalt(II) porphyrin system

36

           

2o. -
.3033 39 -

1.0.0. 32-

cane?
5.2 22-

C: t

C
.2 0.3»?

  

         

0.6.
zone .

 

«Ow. II.

1.0.3 as...

e nu!l
.0».

 
 

 

 

I300 lSOO I700

“00

 

900

RAMAN SHIFT (cm")
RR spectra of (a) CuOEP; (b) CuOEP+'ClO4'. CHzClz bands are marked

with an ’. CW laser power 20-35 mW.

Figure 10.

37

allows us to compare the RR spectral characteristics of metal- vs.
porphyrin- centered oxidized species all with the same metalloporphyrin
complex. Additionally, cobalt spin-state remains constant upon
variation in the ring oxidation and metal ligation states. Thus, the
effects of ring-centered oxidation may be highlighted in the absence of
any complication from changes in the spin-state, the contribution of
which is significant in the RR spectra of metalloporphyrins in the high-
frequency region (1450 - 1700 cm'l). The oxidation products of
biologically-relevant iron porphyrins will be covered in Chapter 3.
Characterization of oxidized iron porphyrins include a wider range of
variables owing to several oxidation, ligation and spin states that are
available to iron atom.

As an appropriate extension of our metalloporphyrin rt-cation
radical interest, we present, in Chapter 4, some preliminary results of the
RR vibrational analysis of metallochlorin n-cation radicals. This study
aims to develop a better understanding of the photosynthetic primary
charge-separating event involving oxidized chlorophylls. Relative to
the Raman spectra of metalloporphyrins, those of metallochlorins are
considerably more complex owing to the symmetry reduction, to
increased macrocycle conformational flexibility and to the likelihood of
changes in normal mode composition of the chlorin macrocycle
vibrations compared to those of the analogous porphyrin.

Preliminary results and future perspectives on two topics that are
still being pursued in our laboratories will be discussed in Chapters 5 and

6. Respectively, they are:

38

1. Novel B-pyrrolic substitution reactions of tetraperfluorophenyl
porphyrin and

2. Metal-axial ligand vibrations in metalloporphyrins, their n-cation
radicals and other related systems - a preliminary vibrational study in
the low-frequency region (< 1,000 cm‘l).

these topics will be presented in this chapter.

10.

11.

12.

13.

14.

15.

REFERENCES

 

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Soc., 1975, Z, 5385.

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Spiro, T. G.; Stong, J. D.; Stein, P., J. Am. Chem. Soc. 1979, 101, 2648.

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743.

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5290.

Mashiko, T.; Kastner, M. E.; Spartalian, K.; Scheidt, W. R.; Reed, C. A., J.
Am. Chem. Soc. 1978, mg, 6354.

(a) Huang, Y. -P.; Kassner, R. J., J. Am. Chem. Soc. 1979, 10_1, 5807.
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Hill, A. O.; Skyte, P. D.; Buchler, J. W.; Leuken, H.; Tonn, M.; Gregson, A.
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Babcock, G. T., J. Phys. Chem. 1987, fl, 5887.

CHAPTER II

PREPARATION METHODS AND SPECTROSCOPIC
PROPERTIES OF 0x11312151) COBALT PORPHYRINS

In contrast to iron and copper, which dominate the scene of transition
metal biochemistry and are components of a variety of metalloproteins, cobalt
occupies a relatively modest position in biology. Although Co(II) porphyrins
do not occur naturally, the related vitamin B12 corrin system catalyzes
molecular rearrangements via the Co(II) oxidation state in at least some of its
enzymatic reactions.1 Most of the literature on cobalt(II) in biochemistry
concerns its effects in various metal-activated enzyme systems.2 Many of the
typical Mg2+, Mn2+, Fe2+ and Zn2+ activated metalloenzymes can work with
Co2+ at a reasonable rate. A general observation seems to be that cobalt
generates catalytic activity, often approaching, and sometimes exceeding that
of the native enzyme.3 Thus, owing to its spectroscopic and magnetic
properties, substitution of Co2+ for the native metal is considered to be a
useful probe of the metalloenzyme active site. The utility of the Coz+ ion is
also well-documented in hemoproteins and their synthetic analogs in which
cobalt substitution has provided additional information for a better under-
standing of spectroscopic, structural and reactivity properties of these
metalloproteins. For example, Co(II)-reconstituted hemoglobins and myo-

globin have been extensively investigated in an attempt to obtain further

43

44

information on heme-heme interations.4 Hoffman and coworkers have
shown that cobalt hemoglobin displays a reversible and cooperative uptake of
oxygen that is qualitatively similar to that of hemoglobin.5 Cobalt porphyrins
have also been applied in disproportionation reactions of hydrogen
peroxideé, which in vivo is prompted by catalase, as "shift reagents" to
determine the stereochemistry of molecules with which they associate7a, and
as the catalyst for various redox reactions of the axial ligands.7b
The study of the oxidation products of cobalt porphyrins is also of great
interest. The versatility of the cobalt porphyrin system allows us to compare
metal vs. porphyrin centered oxidation products for the one-electron case,
and 2A2u vs. 2A1u radicals for the two-electron case, all with the same
metalloporphyrin species. The latter species are of principal interest owing to
their relevance to the Compound I type enzyme intermediates.8 Addi-
tionally, cobalt exhibits only one spin-state (S = 1/ 2) and this can avoid
complications from changes in spin-state upon oxidation, a phenomenon
observed in iron porphyrins.9
It is the aim of this chapter to present first the preparation of oxidized

cobalt porphyrins, followed by their spectroscopic characterization by means
of UV-vis, ESR and magnetic susceptibility, IR and particularly RR technique
to highlight the vibrational properties of porphyrin n-cation radicals. As
noted previously, the pyrrole-substituted porphyrins; i.e., OEP, will be utilized
in this study.
1. Preparative Methods. OEP was synthesized according to the published

method.10 The meso-tetradeuteration of OEP was carried out in D2504-

D20 as described by Bonnett and coworkers.11 CH2C12, freshly distilled

from CaHz, was used as a solvent for all spectroscopic studies unless

otherwise specified.

45

CoHOEP 1: To a refluxing chloroform solution of HZOEP was

added a three-fold molar excess of cobaltous chloride-sodium acetate
(1:1) dissolved in minimum amount of methanol. The progress of the
reaction was followed spectrophotometrically until quantitative
metallation was achieved. The crude reaction mixture was first
evaporated to dryness. The residue was redissolved in minimum
amount of dichloromethane and was washed successively with dilute
acid (once) and with water (several times), to remove excess inorganic
salts, and dried over anhydrous sodium sulfate. The dichloromethane
solution was evaporated to dryness and the resulting fluffy CoOEP(1)
solids were collected. Alternatively, the Co(II) derivative may readily
be obtained from the reaction of free base porphyrin with cobaltous
acetate (10X molar excess) in refluxing acetic acid12 with a similar
workup procedure as described above.

COHIOEPX‘ 2: Method 1. For x- = Br,‘ CoHOEP (30 mg) was
suspended in methanol (30 ml) containing ~ 3 ml of 48% hydrobromic
acid.13 When the suspension was stirred at room temperature for
several hours, the solution gradually changed to a deep red color, until
the red crystals were precipitated. The resulting crystalline precipitates
where collected, washed subsequently with water, methanol,
petroleum ether and dried at room temperature. Yield: > 90%.

Method 2. CoHOEP (50 mg) was dissolved in 10 ml of chloro-
form, and to this solution was added 10 ml of methanol and ~ 0.2 ml of
48% hydrobromic acid.14 The mixture was stirred at room temperature

for 2 h. and then an additional 10 ml of chloroform was added. The

reaction mixture was washed with water, dried over anhydrous
sodium sulfate and evaporated to dryness. Yield: > 90%.

The iodo- and chloro- derivatives may also be prepared as above
by using an appropriate concentration of 57% hydriodic and 37%
hydrochloric acid, respectively.

CoHOEP+~X' 3: This species was obtained by stirring a dry
dichloromethane solution of 1 with a three-fold molar excess of solid
anhydrous silver perchlorate at room temperature for about 1 h.15 The
bright red solution of the neutral CoHOEP turns brownish-red upon
completion. The solution is then filtered and the product can be
isolated by precipitation with hexane. The cobaltous porphyrin cation
radical is stable only in the presence weakly coordinating ligands such
as ClO4", BF4‘ and PF6" but not Br' where the formation of bromo-
cobaltic derivative is favored.

CoIHOEP‘l'QX' 4: When X' = Br', a dichloromethane solution of
1 may be oxidized by means of molecular bromine. The first step,
which requires ~ 0.5 moles of bromine, brings about the oxidation of 1
to 2. Further oxidation of this trivalent complex, using an additional ~
0.5 moles of bromine gives the green cationic radical species 4. When
X- = ClO4", this dibromide species dissolved in dichloromethane is
treated with silver perchlorate16a or, alternatively, a direct two-
electron oxidation of CoHOEP may be achieved in dichloromethane by

using a large excess of ferric perchlorate salt.16b

Com(ROH)ZOEPX’ 5: When R = CH3, this complex was obtained
by oxidation of 1 with HX(X‘ = ClO4',Br‘) in dichloromethane-

methanol

47

(3:1). The diaquo adduct may be prepared in the same manner.
Weakly coordinated methanol and water molecules are readily placed
with stronger ligands such as pyridine.17

4 may also be prepared indirectly by titrative addition of
methanol or water to a dichloromethane solution of CoIIOEP+°X',
ComOEPX‘, and CoHIOEP+-2X' in the order of increasing the required

volume of ROH for complete conversion.18

Instrumentation: UV-vis spectra were recorded by using Shimadzu UV-160
and Cary 219 spectrophotometers. The ESR spectra were measured on a
Varian E-4 spectrometer. The g-value calibration and spin quantitation were
obtained by using Fremy's Saltlsarb Magnetic susceptibility was obtained on
an SHE SQUID susceptometer at a field of SkG. Infrared spectra of Co OEP
compounds were examined as KBr disks by using a Perkin-Elmer 599-IR
spectrophotometer. Raman Spectra were measured with a Spex 1877
Triplemate and OMA H electronics (Oertling et al., J. Phys. Chem. 1987, 9_1_,
5887-5898). Laser emission at 363.8 nm was provided by a Coherent Innova

90-5. Argon Ion Laser.

2. Electronic Absorption Spectra: As described by Johnson”, the various
cobalt porphyrin complexes can be differentiated by their visible spectra
and changes can be followed conveniently by using a spectrophotometer.

The general pattern of a normal pyrrole-substituted metallopor-
phyrin spectra is well known.20 A dichloromethane solution of 1
exhibits such a pattern with two moderately intense bands, the so-called oz

and B bands (8 ~ 104 M'lCm'l) at 551 and 517 nm, respectively, and the

strong Soret band at 391 nm (8 ~ 105 M‘1Cm'1). Of the two visible bands,
the a band has the higher intensity, Figure 1.

. Formation of the Co(III) complex is accompanied by the red-shift of
both Soret and visible peaks with respect to those in Co(II), a
phenomenon pointed out by Corwin and coworkers. They suggested that
this is a general trend for square planar porphyrins bonded to ligands in
the octaehedral positions. The electronic spectrum is always "normal",
but the ratios of the visible on and [3 bands, as well as the exact position of
the Soret band are solvent-dependent to an exceptional degree, Figure 1.
A more recent study by Wang and Hoffman21 support the earlier results
but also demonstrates that the perturbations by the ligand not only shift
the overall spectrum but also systematically change both the frequency
difference between the Soret and the visible bands and the relative
intensity of tea/EB. Thus, the position of the a band shifts less rapidly
than that of the Soret band and the individual axial ligands affect the
spectra in the following order of increasing red shift and decreasing Ea/EB
ratios: "e""(phantom ligand) < H20 <F' <Oz',Im,NMeIm , OH’ < N3‘ <
CN'. This is consistent with the earlier observation that the Soret and the
visible bands of the Co(III) complex are displaced by ~ 20 and 9 nm,
respectively, toward higher wavelength relative to the Co(II) complex and
the relative intensity of the a and [3 bands become nearly equal.

In the optical spectrum of CoHOEP+'ClO4, Figure 2b, the Soret band
occurs at 376 nm and is ~ 1/ 3 as intense as that of ConOEP at 391 nm. The
region between 500-700 nm is broad and featureless, similar to that of

other well-characterized porphyrin it—cation radicals. The contribution of

49

 

I

80 an I

 

SOI-

ABSORBANCE

40

I
\

 

 

 

 

 

 

l
400 500 600 nm

Figure 1. Visible absorption spectra of CoOEP (Soret = 391 nm) and
Com(MeOH)zOEPBr‘ (Soret = 411 nm) in dichloromethane.

50

200 P
f

250

200 -

 

 

ISO

 

elmM“crn")

lOO

 

50

 

 

 

 

WAVELENGTH (nm)

Figure 2. Electronic absorption spectra of CoHOEP derivatives: (a) ConOEP(---); (b)
ConOEP+-CIO4‘(--); (c) ComOEP+'2ClO4‘(....), in CHZCIZ; (d) spectral
changes on addition of methanol to a methylene chloride solution of (b)
producing Com(CH3OH)zOEPClO4‘, total methanol content < 0.01%.

51

3 is apparent in some previously reported optical spectra of
"ComOEPClO4'" and ComOEP+-2ClO4' as a high-energy shoulder on the
Soret absorptions.22 The presence of this radical is overlooked in earlier
reports, most likely because this species is only stable in anhydrous
noncoordinating solvents, i.e., dichloromethane. It seems that an
authentic ComOEPClO4' species, stripped of other coordinating solvents
or ligands, does not exist. Titrative addition of water or methanol to a dry
dichloromethane solution of CoHOEP+' results in appearance of species
with absorption maxima at 409, 522 and 558 nm, Figure 2d. This new
species presumably is Com(MeOH)zOEPClO4' as its optical characteristics
are identical with those of typical Co(III) porphyrin spectra; i.e., the Soret
peak is red-shifted by ~ 20 nm compared with that of 1 and the relative
intensity of the visible bands are decreased. The effect of "wet" solvent is
reversible: if the solvent of the 409 nm species is evaporated to dryness
and the residue is redissolved in dry dichloromethane, the 376 nm species
reappears. This is also demonstrated by a temperature study on
CoIIOEP+' in which an increasing amount of 5 is formed as the
temperature is lowered. As the 376 nm Soret band decreases in intensity,
absorptions characteristic of the cobaltic complex become apparent, Figure
3. Thus, lowering the temperature of this sample has much the same
effect on the absorption spectrum as the addition of methanol,
presumably due to increased diaxial ligation by water with decreasing
temperature. Freeze-pump-thawing to remove water vapor over the
sample prior to cooling minimizes the conversion to the six-coordinate

diaquo cobaltic complex, consistent with diaxial ligation by water.

52

, .5.
376 I

 

Co" 029*” etc; = Com-1,0), 06? etc;

 

ABSORBANCE
M
q
n

 

 

    

 

400 ' 'sdor 600 700
WAVELENGTH (nm)

Figure 3. Electronic absorption spectra showing the conversion of ConOEP+'ClO4‘

to Com(HzO)zOEPClO4‘ as the temperature is lowered. Solvent, "dry"
CHzClz; path length ~ 2mm (EPR tube).

53

The absorption spectra of ComOEPX’(X‘ = Br",Cl',I') appear to be
quite unlike other Co(III) diamagnetic porphyrin spectra due to the
extremely low intensity and rather large blue shift to 373 nm of the Soret
band, Figure 4b. Additionally, the a band shifts to ~ 546 nm with a slight
increase in intensity, while the B band envelope almost disappears.
Because of the anomalous nature of these spectra, a dimeric form has
been suggested for these Co(III) complexes.22 The formation of the
cobaltic porphyrin as a diamagnetic low-spin Co(III) system is supported
by 1H-NMR spectrum which shows sharp lines with no paramagnetic
broadening or contact shifts, Figure 5. The doubling of the a—CHz protons
in the spectrum of ComOEPBr', however, is indicative of the displace-
ment of Co from the plane of the macrocycle. This characteristic has

precedence in the 1H-NMR spectrum of OEPFeHICl, but not in
OEPFeHI(Im)2+ in which a single a-CHZ proton is observed.23 The

diastereotopic nature of -CH2 protons are indicative of the removal of a

plane of symmetry through the methylene protons. While the
counterion X‘ remains bound to the metal in non-coordinating solvents,
it is replaced in coordinating solvents, L, to form the species
Com(L)ZOEPX', Figure 4a, particularly at high concentrations.

There are also distinct changes in the absorption spectra when
CoHOEP is oxidized by two electrons to ComOEP+°2X' (where X' =
ClO4',Br'), Figures 2c and 4c. The optical characteristics of these species as
a function of the counterion have exemplified the two possible radical

ground states and they were described in detail in Chapter 1.

54

200‘

lSO‘

e lmM" cm' 'i
i

 

 

 

WAVELENGTH (nnu

Figure 4. Electronic absorption spectra of oxidation prod-.1cts of CoOEP.
(a) ComMeOl-DZOEPBr'(-);
(b) ComOEPBr‘(--);
(c: ComOEP+'2Br’(-"). the small feature at 40] nm is due to
1% H40£P2+28r' contamination as discussed in the text.
Solvent, drv CH2C12, except for (a) which contains - 5%
methanol (MeOH).

 

 

 

 

-CH)
Hm ‘
2 )UK [IL “2‘ _ J{
I ' l ' 1 1 ' I ‘ fr I l ‘ 1 . ‘. ' I ' to
11.0 10.0 9.0 8.0 7-0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.
PPM

Figure 5. 250-MHZ proton NMR spectrum of ComOEPBr' in CDCl3 at 25°C.
Impurity peaks labeled X.

55

Figure 6 shows that titrative addition of a dichloromethane solution of

the diperchlorate species with methanol also results in the formation of 5.

ESR and Magnetic Susceptibility. The unpaired electron in the low-spin
Co(II) porphyrins is in the dzz orbital, and since the major lobes are not
directed at the porphyrin nitrogen atoms, electron delocalization into the
it orbitals of the porphyrin ring is not allowed. Indeed, the observed
electronic interaction is shown to arise primarily from changes in the
metal-porphyrin 6 bond.24 The unpaired electron, however, is directly

involved in axial interaction and can alter the energy separation between

the dzz orbital and the dxz, dyz orbitals which, in turn, affects the ESR g-
values, i.e., g J_ E 1/ energy gap. Hence the ESR parameters can be used to

identify adducts of Co(II) porphyrins with 1t-or o—donors or acceptors.25

Hambright has reported an observed magnetic moment of 2.89113 for

Cobalt(II) mesoporphyrin-IX dimethyl ester compared with the spin-only
value of 17303.26

Co(III) porphyrin, on the other hand, shows no ESR signal,
consistent with a square-planar 3d6 configuration. An interesting
observation was recently made with cobaltic complexes which are
involved in a disproportionation reaction leading to two paramagnetic
species in equal proportions. Studies by ESR and NMR methods have
indicated that CoHITPPX‘ is converted partially and reversibly into
ComTPP+'2X' and CoIITPP in chlorinated hydrocarbons by thermal or
light activation in aerated solution.2713 The mechanism of the formation

of the radical is initiated by a homolytic cleavage of the Co-X

56

 

 

 

 

 

 

 

A

i
in
0 I
Z
<
D
Q
0
U)
m
<

l I
500 600

WAVELENGTH (nm)

Figure 6. Spectral changes on addition of methanol to a dichloromethane solution of
ComOEP+°2ClO4‘ in the visible region producing Com(CH30H)2OEPClO4‘.
total methanol content ~ 2%.

57

bond to yield CoII porphyrin and chlorin atoms. The subsequent
hydrogen abstraction from the chlorohydrocarbon solvent (i.e., CHzClz)
produces solvent radicals which then react with oxygen molecules to
form oxidant radicals; oxidant radicals may, however, be formed by
photolysis of aerated dichloromethane. These radicals oxidize ComTPPX'

to produce the porphyrin n—cation radical, ComTPPZX‘. The amount of

n—cation radical (~ .5%) that has an 2A2u structure increases in the order

X=I<Br<Cl<PF6<ClO4.

CoHOEP+-Clo4' is ESR silent.15 This indicates that the unpaired
electron of low-spin co11 in the d22 orbital of this radical is spin-coupled
with the porphyrin unpaired electron. The magnetic susceptibility of 3 at
50°K indicates a magnetic moment of 0.215113, most of which is likely to
arise from contaminants for the following reasons:

(a) Upon conversion by additional of methanol of 3 to 5, which is a
diamagnetic d6 CoIII species, the signal persisted with the same
intensity.

(b) Increasing the initial ratio of silver perchlorate resulted in a higher
signal intensity.

(c) The intensity of residual contaminant diminished upon repeated
evaporating-redissolving 3 in dichloromethane. All these observa-
tions suggest the presence of a trace amount (1-2%) of 4 in the
preparation of 3.

The two-electron oxidation product, on the other hand, exhibits a

room-temperature, free-electron-like ESR signal for X’ = ClO4". As in the

CoIIITPP+:2X', spin density is transferred from the oxidized ring onto the

58

metal, and the ESR spectrum can be simulated by using a hyperfine-
splitting constant of ~ 1.2 G for cobalt (I = 7/ 2); this represents a small spin
density on the cobalt ion.28 The dibromide complex has no ESR signal at
room temperature, but as the solution is cooled, a broad signal appears
that narrows with further cooling. Thus, the bromide ligand cause a
significant change in the ESR spectrum. In fact, Dolphin and coworkers
have shown that the bromide ions are so strongly complexed to the di-
cation that the parent peak in the mass spectrum (m/ e 749) is 1/ 10 as
intense as the base peak (CoOEP), whereas the parent peaks of either
FeOEPCl or MnOEPCl are only 1/ 200 as intense as the base peaks (FeOEP
and MnOEP, respectively).28 Thus, the absence of an ESR signal at room
temperature and the broadened nature of the spectrum at low
temperatures suggest a strong coupling interaction between the cobalt and
the cation radical centers caused by conformational and/ or electronic

effects of bromide ion coordination.

Infrared Diagnostic Band of the Cation Radicals: Figure 7 shows the IR spectra
of CoOEP and of its one- and two-electron radical species obtained as the KBr
disk. A feature around 1550 cm-1 increases in intensity in the spectra of the
cation radicals compared with that of the neutral ring compounds. This band
was originally assigned by Shimomura and Goff as the diagnostic band of the
radicals.29 However, dispute as to the authenticity of this band has recently
prompted further studies on a more quantitative basis to expand the
diagnosis of the radicals to other IR bands and to establish vibrational

frequency shifts upon cation radical formation.30 As to the mode composi-

59

 

 

Couoer

 

WWW

T2” '- WW) 0

Co"OEP° 'co:

 

 

 

 

 

 

 

 

 

 

 

Figure 7. Infrared spectra (in KBr) of ConOEP and its oxidized products. The porphyrin
cation radical diagnostic band is labeled with an asterisk.

60

tion of the original diagnostic band, Shimomura g a_l. observed at 10 cm"1
decrease in the frequency of this mode upon meso-tetradeuteration of the

porphyrin macrocycle. If the corresponding modes are correctly correlated in

the -h4 and -d4 species, the isotopic shift is then consistent with the pattern
observed for the vibrations that are predominantly of CaCm stretching

character (V3 and v10) in the RR spectra of metalloporphyrin derivatives

(Section).

RR Vibrational Analysis of CoII OEP and its Oxidized Products-Spectral
Characteristics of the Cation Radicals in the High-Frequency Region (1450-
1700 cm'l: Figures 8-12 show the RR spectra (1000 - 1700 cm'l) obtained with

near -UV excitation of various products of CoOEP oxidation. Comparison of

the vibrational spectra of h4 vs. d4 derivatives of CoOEP, Figure 8a,b, reflects
the involvement of the Cm atoms in the individual normal modes.
Vibrations of CaCm stretching character are expected to exhibit a wave-

number decrease upon substitution of d4 for h4. The bands above 1450 cm"1

are known to reflect porphyrin core geometry and are of principal interest to

this study.
From the mode designations established by Abe g; a_l.,31 the V3, v11, v2

and v10 modes of CoOEP are identified at 1512, 1575, 1599, and 1647 cm-1,
respectively, in Figure 8a. The deuterium shifts of -11 and -8 cm'1 for the
v10(CaCm) and V3 (CaCm) vibrations, respectively, confirm their
compositional assignments. The absence of significant change in the

wavenumbers of the features occurring at 1599 and 1575 cm‘1 in 1 is
consistent with the assignment of v2 (Cbe) and v11 (Cbe) for these

61

Olhe

 

mhn.lllnHII|

he, . 363.8nm

CoOEP

 

 

 

>._._m2m:.ZH

23243.

bid,

 

i300 I500 I700

RAMAN SHIFT (cm")

H00

 

900

Figure 8. RR spectra of cobaltous octaethylporphyrin in CHzClz.

62

vibrations. The weak mode at 1476 cm'1 may correspond to V28 (CaCm), also
reported for NiPP (PP = protoporphyrin-IX) at 1482 cm-1.32 The intense
polarized oxidation-state marker band (V4) appears at 1379 cm'1 in Figure 8a

and is predominantly of CaN stretching character. The depolarization ratio,

the absence of a deuterium shift, Figure 8b, and the clear spectral analogy to
the other well-characterized metalloporphyrins“: 32 forms the basis of this
assignment.

The effects of metal-centered, one-electron oxidation accompanied by

axial metal ligation are demonstrated by comparing the spectra of CoHOEP

and CoIH(MeOH)ZOEPClO4' depicted in Figure 9a,b. The oxidation-state

marker, V4, increases from 1379 to 1383 cm'l, reflecting depopulation of the

porphyrin it" orbitals caused by metal oxidation.33 There is little systematic
change in frequency of modes above 1450 cm'l, indicating that the core size of
the porphyrin ring does not change significantly upon metal oxidation and
addition of axial methanol ligands-32:34

Figure 9c, (1, shows the spectra of the cobaltous and cobaltic OEP+°
complexes. Neglecting differences in relative intensity produced by the
differences in Soret absorption, the RR spectra of 3 and 4 are essentially

identical and (above 1300 cm’l) radically different from those of the neutral

ring compounds in Figure 9a, b. We assign the V3, V11, V2 and V10 frequencies

of CoIIOEP+'ClO4', for example, at 1505, 1604, 1620, and 1642 cm‘l,

respectively, the assignments of which are based upon similarity in

depolarization ratio measurements and isotope data with the authentic 7t-

cation radical ComOEP+-2CIO4', Figures 9c, d and 10.

63

'i379

)‘ex - 363.8 nrn

oi Co" OEP

"' i590

— i375
" 00"

 

- 05.2

   

  
 
 

-t303

bl Co'iMeOHi 206900:
(0 Q

-
2'05
- -

0

- I643

       
 
 
 

i000

RAMAN INTENSITY

c) Co. 0E P" Clo;

I041

- 03“
"' I505

- .54; i6”

 
    

dl Co" OEP” 2cm;

A g A- A. L

Asoo I100 I500 1500 A I7oo

RAMAN SHIFT Icm")

Figure 9. RR spectra excited at 363.8 nm (- 35 mW) of C005? and its oxidation
products. (a) CoOEP; (b) ComMeOI-D20EPC104‘; (c) ConOEP+ClO4’; (d)
Comosr+-2c104-; (e) Com(MeOI-I)20F.P8r'; (0 CoanEPBr‘; (g)

ComOEP+°28r‘. Solvent, dry CH2C12 except (b) and (e) which contain ~
5% methanol. Solvent bands are marked with an ’

64

o
‘2
I

X... - 3535 nm

e) Co"(MeOHizOEP Br'

  

N

O

‘3
t

_. i378

fl Co'OEP Br'

 

 

 

 

 

RAMAN INTENSITY

 

g

.780

900 000 1300 A 1500

RAMAN SHIFT (cm")

Figure 10. Electronic absorption spectra of oxidation products of CoOEP. (a)
Com(MeOH)20EPBr'(-); (b) ComOEPBr' (—): (c) ComOEP+-25r' (---). the small
feature at 401 nm is due to 1% H40EP2+23r‘ contamination as discussed in
the text. Solvent, dry CHzClz, except for (a) which contains ~5% methanol

(MeOH).

65

The R spectra excited at 363.8 nm of the bromide adducts of oxidized
CoOEP are shown in Figure 11. The R spectrum of CoHI(MeOH)zOEPBr',

Figure 11a, is similar to that of CoOEP and Com(MeOH)zOEPClO4', Figure 9a,
b. The R spectrum of COmOEPBr' is shown in Figure 11b. While V4 (1378
cm‘l) is slightly lower than in the other Co(III) compounds, the core-sensitive
bands above 1450 cm‘l, particularly V10 (1657 cm‘l), have increased in
frequency, supporting a slight core contraction in this species.32r34

The spectrum of the it-cationic radical species, ComOEP+°2Br'(Figure
11c) is dominated by v2 (1611 cm-1) and v11 (1600 cm-1) modes, similar to the
other n-cations discussed earlier. There is, however, no band easily assignable
to V4. The features at 1460 and 1648 cm"1 are assigned to V28 and V10,

respectively. The V3 vibration, which is enhanced in Soret RR spectra of

neutral metalloporphyrins, does not appear in these spectra, but a polarized
feature at 1497 cm'1 present in the spectra obtained in resonance with the 670-
nm transition of the dibromide cation radical is assigned to V3. (Spectrum
not shown).18

Table 1 summarizes the vibrational frequencies of CoOEP derivatives.
Thus, in the high-frequency region, the modes involving primarily CaCm
stretching character (V3 and V10) decrease in frequency upon oxidation of the
porphyrin ring, while modes involving primarily Cbe stretching character
(V11 and V2) increase in frequency. The frequency of V4, primarily a CaN

stretch, decreases upon formation of porphyrin rt-cation radical. These trends
are common to other metalloporphyrin system (i.e., CuOEP, ZnOEP) not

presented here.18

66

4

m
C
2
0
P
E
0
l
O
C

 

)‘u - 363 8 nm

e
h
g

i

a) "4

D) de

>P_m2u._.ZH Z<2<m

C) he

 

de

d)

 

I500 I700
RAMAN SHIFT (cm")

l300

iIOO

 

900

Figure 11. RR spectra of cobaltic octaethylporphyrin it cation radicals in CHzClz.

67

Table 1: Resonance Raman Frequencies (cm'l) for the Parent CoOEP and its Oxidation Products

 

 

V4 V3 V11 v2 v10 Soret, nm
C aN Cacm Cbe Cbe CaCm
CoIIOEP-h4 1379(2) 1512(2) 1575(6) 1599(3) 1647(0.7) 391
d4 1379(3) 1504(2) 1572(9) 1598(4) 1636(1.0)
CoIII(MeOH)20EPx‘
x' =ClO4' 1383 1515 1574 2 1645 409
Br' 1381 1515 1575 1599 1648 411
ComOEPBr 1378 1515 1577 1601 1657 373
CoUOEP+Clo4‘ 1363 1505 1604 1620 1642 376
CoIIIOEP+-2x-
x' = ClO4‘,h4 1360(3) 1505w 1605(8) 1617w 1642 sh392
d4 1360(4) 1496(6) 1603(8) 1618sh 2
x‘ = Br’,h4 2 1497(3) 1603(5) 1611(4) 1649(7) 410,
d4 2 2 1603 1611 1639 344

 

Depolarization ratios, given in parentheses, were also measured with
363.8-nm excitation.

Perhaps the most interesting aspect of the RR spectra of ComOEP+-2X'

is the apparent insensitivity of the RR frequencies to radical electronic state.

Aside from differences in relative intensities, the RR spectra of the two

radicals are similar; however, the vibrational frequencies of the dibromide

complex (ZAlu) are 6-7 cm‘1 lower than those of the diperchlorate (2A2u),

with the exception of v10 with no explanation offered as yet. Because the

68

frequency differences are small, we speculate that they can be attributed to
differences in porphyrin core geometry caused by the different axial ligation of
these two species. Although these vibrations of the porphyrin macrocycle
may well be insensitive to the radical electronic state, the abnormally high

values of the V(Cbe) modes provide a clear basis to identify ring-centered

oxidations.15

CoIHOEP+°2ClO4' is the only species that remains identified in the
literature as a 2AZU radical among the MOEP+° complexes, thus, our sugges-
tion regarding the insensitivity of porphyrin RR vibrations to the radical
electronic state is not strongly supported. Since the ESR data is inconclusive
as to the ZAZU assignment of the diperchlorate adduct (e.g., lack of Co hfs), it
may be that this species belongs to a 2Aw ground state as well. Reed and
coworkers have recently stressed the importance of the evaluation of
porphyrin stereochemistry as well as n-orbital occupancy for the analysis of
the magnetic properties of metalloporphyrin it-cation radicals. Thus, the
structural factors, in our opinion, may also play a significant role in the
analysis of the electronic and vibrational spectra of these complexes. It is
likely that the absorption spectra of CoIIIOEP+'2ClO4‘ and CoIIIOEP+'2Br',
which were originally thought to be characteristics of the 2A2U and ZAIU
states, respectively, actually reflect the stereochemistry of the porphyrin core
and that a planar conformation exists in the solution for the diperchlorate
complex, and presumably a nonplanar one is present for the dibromide
adduct. The RR spectra may in turn, reflect the structural properties of these

radicals and, that they are either to some extent independent of 2A1U vs. 2AZU

69

designation or that all of the MOEP+° compounds we have examined belong
to the 2Aw ground state. In order to resolve questions regarding structure we
are currently studying the low-frequency RR active vibrations of the metal
axial ligand and porphyrin skeleton of these complexes. In addition, EN DOR
measurements are being used to determine radical ground state assignment.

We will report on these efforts in the future.

10.

11.

12.

13.

REFERENCES

Dolphin, D., Ed., B12 (Wiley, New York, 1982), Vols. 1 and 2.

LindsKog, S., Struct. Bonding (Berlin) 1970, 8, 153-196.

Smith, E. L.: In: Mineral Metabolism 2, Part B, 349. Ed. by C. L. Comar
and F. Bronner. New YorkuLondon: Academic Press, Inc. 1962.

Yonetani, T.; Yamamoto, H.; Woodrow, G. V., J. Biol. Chem. 1974, 249,
682.

HSU, G. C.; Spilburg, c. A.; Bull, C.; Hoffman, B. M., Proc. Natl. Acad. Sci.,
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Lee, W. A.; Bruice, T. C., Inorg. Chem. 1986, 25, 131.

(a) Hill, H. A. O.; Sadler, P. J.; Williams, R. J. P.; Berry, C. D., Ann. N .Y.
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Hanson, L. K.; Chang, C. K.; Davis, M. S.; Fajer, J., J. Am. Chem. Soc., 1981,
fl, 663.

Scholz, W. F.; Reed, C. A.; Lee, Y. J.; Scheidt, W. R.; Lang, G., J. Am. Chem.
Soc., 1982, 1% 6791.

Wang, C. B.; Chang, C. K., Synthesis, 1979, _54_8.

Bonnett, R; Gale, I. A. D.; Stephenson, G. F., J. Chem. Soc. (c), 1967, 1168.

Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J., J. Inorg. N ucl. Chem., 1970,
Q, 2443. .

Sakurai, T.; Yamamoto, K.; Naito, H.; Nakamoto, N., Bull. Chem. Soc.
Jpn. 1976, 49, 3042.

70

14.

15

16.

17.

18.

19.

20.

21.

24.

26.

27.

28.

29.

71

Datta-Gupta, N .; Bardos, T. J., J. Pharm. Sci. 1968, 52, 300.

(a) Salehi, A.; Oertling, W. A.; Babcock, G. T.; Chang, C. K., J. Am. Chem.
Soc., 1986 fl, 5630.

(b) Wertz, J. E.; Reitz, D. C.; Dravneiks, F. In "Free Radicals in Biological
System," Blois, Jr., M. 5., Ed., Academic: New York, 1961, _1_85.

(a) Dolphin, D.; Muljiani, Z.; Rousseau, K.; Borg, D. C.; Fajer, J.; Felton, R.
H. Ann. N. Y. Acad. Sci., 1973, @5, 177.

(b) Groves, J. T.; Quinn, R.; McMurry, T. J.; Lang, G.; B050, 8., J. Chem.
Soc., Chem. Commun., 1984, 1455

Sugimoto, H.; Ueda, N .; Mori, M., Bull. Chem. Soc. Jpn. 1981, 54, 3425.

Oertling, W. A.; Salehi, A.; Chung, Y. C.; Leroi, G. E.; Chang, C. K.;
Babcock, G. T., J. Phys. Chem., 1987, 21_, 5887.

Johnson, A. W.; Kay, 1. T., J. Chem. Soc., 1960, 2979.

Edwards, L.; Dolphin, D. H.; Gouterman, M. J., Mol. Spectrosc., 1970, 55, 90.
Wang, M. -Y. R.; Hoffman, B. M., J. Am. Chem. Soc., 1984, m6, 4235.
Browett, W. R.; Stillman, M. J., Inorg. Chim. Acta, 1981, 42, 69.

(a) Walker, F. A.; La Mar, G. N. Ann. N. Y. Acad. Sci., 1973, E64 328.
(b) LaMar, G. N .; Walker, F. A., J. Am. Chem. Soc., 1973, g, 1782.

Walker, F. A., J. Mag. Res., 1974, _1_5, 201.
Fulton, G. P.; LaMar, G. N ., J. Am. Chem. Soc., 1976, 25, 2119.

Hambright, P.; Thorpe, A.; Alexander, C. J. Inorg. Nucl. Chem., 1968, 3Q,
3139.

(a) Huet, J.; Gaudemer, A.; Boucly-Goester, C.; Boucly, P., Inorg. Chem.,
1982, 21, 3413.

(b) Yamamoto, K.; Hoshino, M.; Kohno, M.; Ohya-Nishiguchi, H., Bull.
Chem. Soc. Jpn., 1986, 52, 351.

Dolphin, D.; Forman, A.; Borg, D. C.; Fajer, J.; Felton, R. H., Proc. Natl.
Acad. Sci. USA, 1971, 55 614.

Shimomura, E. T.; Phillippi, M. A.; Goff, H. M., J. Am. Chem. Soc., 1981,
E3, 6778.

30.

31.

32.

33.

34.

72
(a) Itoh, L.; N akahasi, K.; Toeda, H., J. Phys. Chem., 1988, 22, 1464.
(b) Hinman, A. S.; Pavelich, B. J.; McGarty, K., Can. J. Chem., submitted.
Abe, M.; Kitagawa, T.; Kyogoku, Y., J. Chem. Phys., 1978, 52m 4526.

Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, D. L.; La Mar, G.
N ., J. Am. Chem. Soc., 1982, _1_24, 4345.

Spiro, T. G.; Strekas, t. C., J. Am. Chem. Soc., 1974, 25, 338.

Spiro, T. G., In "Iron Porphyrins," Lever, A. B. P.; Gray, H. B., Eds.;
Addison-Wesley: Reading, MA, 1983; Part 2, 89.

CHAPTERIII

IRON PORPHYRIN l'I-CATION RADICALS:
SOLUTION RESONANCE RAMAN SPECTRA OF

OEP+°Fem(X)(X')

Heme catalysis in peroxidases, catalases and cytochromes P-450
proceeds through high valent intermediates, some of which are likely to be
oxidized at the porphyrin macrocycle as well as the central iron.1 An
understanding of the role of these species in the enzymatic cycle requires
insight into the interplay between the oxidation state, geometry and electron
density distribution of the porphyrin ring and the valence, spin and ligation
states of the iron.2 Raman spectroscopy is potentially able to provide this
kind of information and its utility was demonstrated in the previous chapter
in the detection of cobalt porphyrin oxidation products.3 This work,
however, was confined only to the cobalt complexes in which the spin state
remains unchanged upon variation in the ring oxidation and metal ligation
states. Characterization of oxidized iron porphyrins, on the other hand,
covers a wider range of variables owing to several oxidation, ligation and spin
states that are available to the iron atom, and has not yet been achieved by
Raman spectroscopy.4

For example, for ferric porphyrins, which serves as the neutral

precursor in this study, the spin-state and stereochemistry of the iron(III)

73

74

center are controlled by the nature of axial ligands.5 The coordination of two
strong field ligands leads to low-spin (S = 1/ 2) six-coordinate hemes, e.g.,
bis(imidazole) iron(III) derivatives. Weaker field ligands, typically anionic
ones such as chloride and azide, lead to five-coordinate high-spin (S = 5/ 2)
derivatives. The high-spin five-coordinate species have the iron(III) atom
displaced out of the porphyrin plane by ~ 0.5A with concomitant longer Fe-N
bonds. The low-spin, six-coordinate complexes have the iron(III) atom
essentially centered in the porphyrin plane and relatively short Fe-N bonds.
The stereochemical differences are associated with the population of the
3dx2-y2 orbital in the high-spin complexes and its depopulation in the low-
spin derivatives. The coordination of two weak field ligands to the ferric
porphyrins leads to still another structural type for iron(III) porphyrins;
namely, high-spin six-coordination. Ligands which lead to this type are
commonly neutral oxygen donor ligands: sulfoxide, dimethylformamide,
pyridine N-oxide, triphenylphosphine oxide and water.

When the axial ligands are extremely weak anionic ligands, such as
perchlorate or tricyano methanide [C(CN)3'], the resulting ferric porphyrin
derivatives are not high-spin. Rather these complexes have magnetic
properties and stereochemical features consistent with a ground-state
assignment of intermediate (S = 3 / 2) or a quantum mechanical admixture of
S = 3/ 2 and S = 5/ 2 spin state.6 This formulation is to be distinguished from a
thermal mixture of magnetically distinct S = 3 / 2 and S = 5/ 2 states. Quantum
mechanical admixing is presumably the result of small energy separation
between pure spin states.7 For the iron (HI) porphyrin perchlorate derivative,

the molecular structure reveals a shortened Fe-N distance, a small Fe-out-of

75

plane displacement (0.3A), and a lengthened Fe-perchlorate oxygen distance
with respect to high-spin derivatives.8
To address these properties at the oxidized level, we report here the

solution RR characterization of iron(III) octaethylporphyrin cation radicals of

general formula OEP+°FeHI(X)(X'), where X' = Cl', ClO4", and X' = ClO4' and

SbClg‘. We focus on the determination of the solution coordination number

and spin state of the iron atom in the porphyrin radicals, as monitored by
high-frequency (1450 - 1700 cm’l) vibrational modes that are sensitive to
porphyrin core conformation. Our data demonstrate the utility of Raman
spectroscopy in characterizing iron porphyrin radicals and indicate a
preference for the five-coordinate state in solution, at least for the ligand
combinations used here. We discuss our results in light‘of recent NMR,
crystal structure, magnetic, and IR data available on the related TPP

derivatives.
EXPERIMENTAL SECTION

Instrumentation. UV-vis spectra were recorded by using a Cary 219
spectrophotometer. The 363.8 and 351.1 nm lines used to record the RR
spectra were obtained from a Coherent Innova 100-20 argon ion laser; the
Raman equipment is described in detail elsewhere.3C Magnetic susceptibility
measurements were performed on an SHE SQUID susceptometer at a field of

5KG.

Materials and Methods. The free base porphyrin HZOEP and HZEPI

(etioporphyrin-I) were synthesized according to the published methods.9r10

76

Iron insertion was achieved by using ferric chloride Salt.11 Exchange of

hydrogen (h4) for deuterium (d4) at the meso position of the porphyrin was
accomplished by using the Dst4/DZO method and confirmed by NMR

measurements. 1 2

Chemical Oxidations. Typically, samples were prepared at room temperature
in dry CH2C12 by using a slight stoichiometric excess of the oxidant, according
to the published reportszar13 The progress of the reaction was followed
spectrophotometrically until complete oxidation was achieved.
OEP+°Fem(ClO4')2 (1) was prepared from (OEPFe)2O by using Fe(ClO4)3-tzO
(yellow) in the presence of trace HClO4 to assist in the breaking of the u-oxo
bridge of the dimer. UV-vis (CHzClz) Amax (8x10‘3) for 1 are as follows: 357
(86), 517 (14.5), 578 nm (broad), Figure 1; ueff = 6.7113 (solid state, 300K).
OEP+'Fem(Cl')(SbC16’) (2) was prepared from OEPFeCl by using phenoxathiin

hexachloroantimonate as the oxidant.2C UV-vis (CHzClz) lmax (8x10‘3) for 2
were similar: 357 (84), 519 (14.7); lieff = 2113 (solid state, 300K).

Integrity of the samples was verified by monitoring the absorption
spectra before and after the RR spectra. The choice of the excitation line,
however, is of importance to minimize contributions from ferric porphyrin
and diacid salt contaminations,3b which may be difficult to observe in the

optical spectra, and to select predominantly for the porphyrin radical.

Results and Discussion. Our magnetic susceptibility data for 1 (ueff = 6.7113)

and 2 (ueff = 5.2113) show that the effective magnetic moment for ferric OEP 1t-

cation radicals in the solid state is a function of the axial ligation. A similar

dependence occurs for solid state ferric TPP radicals as TPP +'Fem(ClO4')2 (3)

77

 

 

 

 

IOO r r r I I i l
357
80* ..
i“ 60 -
E I'
-U
I )— _r
E
E
- 4o- -
W
.. 12 .1
20" 520 -I
a L A P L A A 1 A A a L a a L
300 400 500 600 700

Figure 1.

WAVELENGTH (nm)

Absorption spectrum of FemOEP+°(Cl')(SbC15') in CI-lzClz.

78

has neff = 6.5 while TPP+-FeIH(Cl')(SbC16') (g) has lieff = 4.8ttB.14 The latter

data have been rationalized by X-ray crystallographic work.14115 3 has a
planar core with both perchlorate ions ligated to the iron; this geometry
apparently leads to strong ferromagnetic coupling between the high-spin
ferric ion and the ring-centered radical to produce an overall spin of S = 3.
Solid state IR data on. 3 and its TMP analog are consistent with bisaxial
ligation.15,16 4, however, is five-coordinated and exhibits a ruffled core,
which presumably leads to antiferromagnetic exchange and a room
temperature 5 = 2 state.

In solution 4 maintains its five-coordinate ligation geometry. Boersma
and Goff, however, have interpreted NMR and magnetic data to suggest that
3 undergoes axial ligand dissociation and also adopts a five-coordinate
geometry in solution.” Raman spectroscopy of these ferric cation radicals
provides an independent means by which to test this suggestion; moreover
such a study will assess the applicability of Raman techniques to
characterizing ring-oxidized iron porphyrin species. As in our earlier work,
we have focused on iron OEP derivatives owing to their biologically relevant
peripheral substitution pattern. The similarity in magnetic moments
between FeIHOEP cation radicals and FemTPP cation radicals noted above
indicates that the altered pattern of peripheral substitution is unlikely to
perturb axial ligation behavior significantly.

Figure 2 shows Raman spectra of 1 and 2 along with that of the five-
coordinate ring neutral OEPFem(C1‘) species (é) in methylene chloride. An

assignment of the principal high frequency modes for these species as well as
of OEPFem(DMSO)2+ (Q18 is given in Table I. Compositional assignment

Figure 2.

79

   
    

l3!

cl OEPFcnlCl‘)
363.8 MI

 

b) OEP? F300;)2
363 8 nm

   

  

RAMAN INTENSITY

36“ nm

 

c) oer? Fe'to'llsaop
363.8 nm

 

RAMAN SHIFT km")

Near-UV RR spectra of OEP+°Fem(X)(X') complexes. The laser power
was 3540 mW. The solvent (CHZClz) band at 1423 cut-1 is labeled with

an asterisk (').

80

for the individual normal modes has been confirmed by a combination of
meso-deuterium isotope effect and spectral comparison between the
analogous OEP and EPI complexes, Figures 3 and 4.19 The vibrational modes
with significant CaCm stretching character are expected to exhibit a decrease
in wavenumber upon meso-tetradeuteration. On the other hand, the
frequencies of vibrations that are predominantly of Cbe character should

remain unchanged. The contributions from Cbe stretching motions may,

however, be more directly confirmed by comparing the spectra of OEP vs. EPI
complexes, which also serves as an alternative to isotopic labelling at the B-
pyrrole positions. The frequency differences apparent in comparing
corresponding modes for the two ring neutral species, 5 and 6, are understood
and correlate inversely with porphyrin core size. The similarities for 1 and 2
in the frequencies of these modes, which retain core-size sensitivity in the
metalloporphyrin cation radicals,3C indicate that the solution geometry for
these two radicals is similar, in agreement with the Boersma and Goff
suggestion.

We can gain further insight into the coordination state of these radicals
by referring to earlier work with valence +2 metalloporphyrin derivatives.3C
For this class of compounds we showed that vibrational modes with

predominantly Cbe character are ~20 cm"1 higher in a given radical relative

to the corresponding modes in its neutral precursor, whereas those with
significant CaCm character are ~5 cm'1 lower (Table 1, column 6). The
physical basis for this behavior lies in the fact that there is a little change in
porphyrin core geometry upon ring-centered oxidation. This correlation

indicates, then, that the axial coordination for the OEP radicals in Figure 1

81

(ca ' l #3059

 

an. I

A" 0 363.8 nm

0) fig

 

 

b) d.

 

 

 

 

 

> :mZWFZH 242$”.

 

'300 1500 woo

woo

 

RAMAN SHIFT tern")

RR spectra of ferric chloride porphyrins in CHzClz.

Figure 3.

82

 

 

>2m2uhZH Zdidm

 

ITOO

 

RAMAN SHIFT km")

RR spectra of ferric chloride porphyrin 1c cation radicals in CHzClz

Figure 4.

83

may be assigned by considering their frequency shifts relative to the neutral
five- and six-coordinate ferric OEP species. Inspection of Table I shows that
the pattern of frequency shifts observed for the valence +2 metalloporphyrin
derivatives is reproduced when the ferric n-cation radicals are considered to
be five—coordinate in solution (column 4); assuming a six-coordinate solution
state for the radicals (column 5) produces poor agreement with our earlier
data.

Two conclusions follow from these data. First, the porphyrin core
geometries of both 1 and 2 in solution are characteristic of a five-coordinate
state, consistent with the suggestion that axial ligand dissociation occurs
when 2 goes into solution.17 Second, the pattern of vibrational frequency
changes that occurs upon ring oxidation of +2 metalloporphyrin
derivatives3C appears to hold as well for the more complex, but biologically

more relevant, iron porphyrins.

High frequency RR scattering from Iron protoporphyrin-IX lt-cation radical:
Figure 5 depicts the RR spectra of OEPFeCl and its one-electron
oxidation product, OEP+°Fem(Cl')(SbCl6’). Also shown are the spectra of the
analogous protoheme species (Figure Sc and d). As demonstrated earlier, the
OEP compounds exhibit a five-coordinate, high-spin (S = 5/ 2) configuration
for the heme iron in solution; the protoheme spectra are consistent with the
same spin and coordination state assignment for these species.20 Table 2
compares the vibrational assignments of iron PP-IX with those of OEP
complexes. The decrease in frequency for modes with significant Cbe
character for the protoheme species relative to OEP species is expected on the

basis of different pattern of peripheral substitution, also demonstrated in the

spectra of the EPI complexes. The protoheme RR spectra are more complex

owing to the Eu modes, V33 and V37, which occur at 1530 and ~ 1553 cm'l,

respectively, in the neutral form, Figure 5021 The position of V38 in the

cation radical is difficult to determine from our RR spectra excited at 351.1
nm, Figure 5d, but presumably can be measured by IR spectroscopy.3C We

interpret the broad feature extending from 1570 - 1600 cm'1 in the FeHIPP-IX+°
spectrum to be composed of three overlapping bands: V11 (Cbe) occurring at

~ 1570 elm-1, v2 (Cbe) at 1593 cm-1 and V37 (CaCm) at ~ 1585 cm-1. The ~ 20
cm'1 increase in the modes (V11, V2) that are predominantly composed of
Cbe stretches is expected upon oxidation of ferric porphyrin lt-cation radical,
Table 1. These frequency increases are partially obscured in the spectrum in
Figure 5d by the frequency decreases of V37 (CaCm) in the cation (~ 1585 crn‘l)
relative to the neutral (1590 cm‘l). These spectra demonstrate that the
changes in the vibrational frequencies that accompany ring oxidation of these
ferric compounds are consistent with those observed for other cation radical

species. Thus, in general, we find that stretching modes with predominantly

CaN character (V4) or CaCm character (V3 and V10) decrease in frequency,

while those with (3be character (V11 and V2) increase in frequency when the

porphyrin ring is oxidized, Tables 1 and 2. We see these frequency shifts

regardless of whether the electron is extracted from the a1u(7c) or a2u(1t)

molecular orbital.

8S

 

 

 

 

>._._mZm:.ZH Z<§<m

 

RAMAN SHIFT (cm")

RR spectra of model compounds in CHZClz Conditions: 2530 mW

incident on the sample in a spinning cell at 25°C.

Figure 5.

86

Table 1.
RR Frequencies (cm-1) of Selected Iron OEP Species

 

 

OEPFeIII OEPFem
osr+-Fe111 OEP+'Fem (Cl') 5 (0M50)2+ 6 Predicted
Mode (ClO4')2 1 (Cl’)(SbCl6‘) 2 (Mia) (Av)a) Avaveb)
v10(cacm) 1626 1627 1629 (--3) 1615 (+11) .4
v2(Cbe) 1602 1602 1581 (+21) 1576 (+26) +20
V11(Cbe) 1576 1577 1560 (+21) - +26
v3(cacm) 1490 1487 1496 (-7) 1483 (+6) -7

 

a) Obtained as the average frequency difference for the indicated mode in the cation radicals

and either the neutral five-coordinate species (column 4) or the neutral six-coordinate
species (column 5).
b) From ref. 3c.
Table II.

Vibrational Assignments (1450-1700 cm’l)
for Ferric Porphyrins and their 1): Cation Radicals

 

ClFeOEP ClFeOEPT' C1 FePPIX C1 FeI’PIXT‘

 

V3(Cacm) 1494 1486 1493 1490
V11(Cbe) 1559 -1577 1553 -1570
v2(cbcb) 1581 1602 1572 -1593
V37(Cacm) - - 1590 -1585
v10(cacm) 1629 1625 1630 1628

(a)
(b)
(C)

(a)
(b)

(C)

(d)
(e)

(f)

(a)
(b)
(C)
(a)
(b)
(C)

REFERENCES

Edwards, S. L.; Xuong, N. H.; Hamlin, R. C.; Kraut, J. , Biochemistry,
1987, _2_6, 1503.

Hanson, L. K.; Chang, C. K.; Davis, M. S.; Fajer, J., I. Am. Chem. Soc.,
1981, m 663.

Guengerich, F. P.; MacDonald, T. L., Acc. Chem. Res., 1984, _1_7.

Phillippi, M. A.; Goff, H. M., I. Am. Chem. soc., 1982, 1% 6026.
Phillippi, M. A.; Shimomura, E. T.; Goff, H. M., Inorg. Chem., 1981,
_2_Q, 1322.

Gans, P.; Marchon, I.-C.; Reed, C. A.; Regnard, I. -R., Nouv. I. Chim.,
1981, _5_, 203.

Furhop, I. H., Struc. Bonding (Berlin), 1974, E, 1.

Groves, I. T.; Quinn, R.; McMurry, T. J.; Lang, G.; Boso, B., J. Chem.
Soc. Chem. Comm., 1984, 1455.

Erler, B. 5.; Scholz, W. F.; Lee, Y. J.; Scheidt, R.; Reed, C. A., I. Am.
Chem. Soc., 1987, 1_09, 2644.

Salehi, A.; Oertling, W. A.; Babcock, G. T.; Chang, C. K., I. Am. Chem.
Soc., 1986, 1_0_8_, 5630.

Oertling, W. A.; Salehi, A.; Chang, C. K.; Babcock, G. T., J. Phys.
Chem., 1987, 21, 3114.

Oertling, W. A.; Salehi, A.; Chung, Y. C.; Leroi, G. E.; Chang, C. K.;
Babcock, G. T., J. Phys. Chem., 1987, _9_l_, 5887.

Goff, H. M.; Phillippi, M. A., J. Am. Chem. Soc., 1983, m, 7567.

Lee, W. A.; Calderwood, T. S.; Bruice, T. C., Proc. Natl. Acad. Sci. U.S.,
1982, 324 4301.

Balch, A. L.; Latos-Grazynski, L.; Renner, M. W., I. Am. Chem. Soc.,
1985, m, 2983.

Sheidt, W. R. In The Porphyrins, Dolphin, D. H. Ed., Academic press:
New York, 1978, Vol. 3, 463.

87

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

88

Dolphin, D. H.; Sams, I. R.; Tsin, T. B., Inorg. Chem., 1977, E, 711.
Goff, H. M.; Shimomura, E. T., I. Am. Chem. Soc., 1980, 10_2_, 31.

Reed, C. A.; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, W. R.;
Spartalian, K.; Lang, G., I. Am. Chem. Soc., 1979, 1_Q1, 2948.

Wang, C. 8; Chang, C. K., Synthesis, 1979, 548.

Fuhrhop, I. -H.; Smith, D. M. in Porphyrins and Metalloporphyrins,
Smith, K. M., Ed., Elsevier: Amsterdam, 1975, 765.

Chang, C. K.; Dinello, R. K.; Dolphin, D., Inorg. Synth., 1980, Q, 147.

Fuhrhop, I. ~H.; Smith, K. M. in Porphyrins and Metalloporphyrins,
Smith, K. M., Ed., Elsevier: Amsterdam, 1975, 816.

Carnieri, N ., Harriman, A., Inorg. Chim. Acta, 1982, Q, 103.

Gans, P., Buisson; G., Duee, E.; Marchon, I. -C.; Erler, B. 8.; Scholz, W. F.;
Reed, C. A., I. Am. Chem. Soc., 1986, 1%, 1223.

(a) Scholz, W. F.; Reed, C. A.; Lee, Y. J.; Scheidt, W. R.; Lang, G., I. Am.
Chem. Soc., 1982, 1_04, 6791.

(b) Buisson, G.; Deronzier, A.; Duee, E.; Gans, P.; Marchon, I. -C.;
Regnard, I. -R., I. Am. Chem. Soc., 1982, 1_0_4, 6793.

(c) Groves, I. T.; Quinn, R.; McMurry, T. J.; Nakamura, M.; Lang, G.;
Boso, B., I. Am. Chem. Soc., 1985, L07, 354.

WARNING: Laser irradiation of solid perchlorate salts may cause
explosion.

Boersma, A. D.; Goff, H. M., Inorg. Chem., 1984, 2_3, 1671.
Callahan, P. M.; Babcock, G. T., Biochemistry, 1981, 20, 952.

Oertling, W. A.; Salehi, A.; Chang, C. K.; Babcock, G. T., J. Phys. Chem.,
submitted.

Oertling, W. A.; Babcock, G. T., Biochemistry, in press.

Choi, 5.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, D. L.; LaMar, G.
N., I. Am. Chem. Soc., 1982, 1%, 4345.

CHAPTER IV

RESONANCE RAMAN SPECTRA OF THE

H-CATION RADICALS OF COPPER, COBALT,
AND NICKEL METHYLOCTAETHYLCHLORINS:

VIBRATIONAL CHARACTERISTICS OF
CHLOROPHYLL MODELS

The primary charge-separating event in photosynthetic higher
organisms proceeds via the light-induced generation of chlorophyll a radical
ions in the reaction centers of Photosystems I and 11.1:2 The nature of the
resulting oxidized chlorophyll species has been controversial and both
monomer and dimer formulations have been suggested for P-700+ in
Photosystem I and P-680+ in Photosystem 11.3145 Lutz and coworkers have
provided Raman data that suggest that the radical may be localized in the
related bacterial system, at least on the vibrational time scale.6

Although there is uncertainty in the extent of the delocalization of the
unpaired electron, it is clear that the chlorophyll species is well-suited to its
role in photosynthetic systems. A major factor in this function is the relative
ease of chlorophyll cation radical formation. This is primarily due to the fact
that chlorophyll contains a chlorin ring as its basic structural feature and
metallochlorins are oxidized at potentials that are as much as 0.3V lower than

those of their metalloporphyrin counterparts.718r9

89

90

A variety of spectroscopic methods have been used to characterize
metallochlorin n-cation radicals, including UV-vis, ESR, EN DOR, and cyclic
voltammetry-7I3I10111r12 These methods have provided information
regarding the optical properties, electron-density distribution, ground-state
assignments, and redox properties of the radical species; however, their
vibrational characteristics have not been explored in any detail. This may
partially result from the limited stability of the one-electron oxidized
metallochlorin system and its tendency to convert to the corresponding
metalloporphyrin derivative“):12 Stability of these radicals may, however,
be enhanced if one of the substituents on the reduced ring is an alkyl group.
This substitution inhibits oxidative dehydrogenation of the chlorin ring to
porphyrin, a reactiOn common to chlorins in which the reduced ring contains
vicinal protons.13 In this regard, methyloctaethylchlorin (MeOEC), Figure 1,
represents a simple form of the alkyl chlorins that is ideally suited for
oxidation study; moreover, its structure closely resembles the more
commonly-studied cis/ trans octaethylchlorin (OEC). Additionally, an alkyl
chlorin has been suggested as the prosthetic group in the catalytic cycle of
Neurospora crassa catalase.14 Earlier, we demonstrated the application of RR
spectroscopy for the vibrational characterization of metalloporphyrin ic-cation
radicals, MP+°15r16 We present here the RR spectra obtained with excitation
at 363.8 nm of Cu(II), Co(II) and Ni(II) complexes of MeOEC and their n-cation
radicals. Additionally, the Raman spectra of the Cu(II) derivatives of t-OEC
are presented for comparison. This study aims to compare the vibrational
characteristics in the high-frequency region (1450—1700 cm'l) of the oxidized
metallochlorins (MC+') to those of the corresponding metalloporphyrin 1t-

cation radical (MP+') derivatives.

91

 

(0) t-CUOEC (b) CuMeOEC

Figure 1. Chemical structures of copper(II) derivatives: (at) trans-octaethylchlroin
(t-OEC); (b) methyloctaethylchlorin (MeOEC).

92

Materials and Methods. t-HzOEC and HzMeOEC were prepared according to

the published methods.17:18 Metal insertion was carried out by adding a
saturated CH3OH solution of M(OAC)2'XHzO-NaOAC to a CHzClz solution

of the free base chlorins. Metallochlorins were oxidized chemically at room
temperature according to the AgClO4/CH2C12 method.19 The progress of the
reaction was followed spectrophotometrically until complete oxidation was
achieved. The color change is diagnostic: the initial purple-blue (Cu) or
green (Co,N i) color of the neutral chlorin complex changes to yellow.

Absorption spectra were measured on Shimadzu UV-160 and Cary 219
spectrophotometers. The 363.8 nm line used to record the RR spectra was
obtained from a Coherent Innova 100-20 argon ion laser, the Raman
equipment is described in detail elsewhere.15 '

The integrity of the samples was verified by monitoring the absorption
spectra before and after recording the Raman spectra. Cu(II), Co(II), and N i(II)
complexes of MeOE , as well as that of t-CuOEC, were sufficiently stable
under aerobic conditions, and for this reason samples were not routinely
degassed. The corresponding cation radicals were also tractable over the
course of RR study with minimal decomposition. On prolonged irradiation,
t-CuOEC+° decomposed to CuOEP (Soret = 397 nm) for the most part, whereas

MeOEC+° complexes returned to the neutral state.

Results and Discussion. Optical spectra of CuMeOEC and its one-electron
oxidation product are shown in Figure 2. The cation radical exhibits a split
Soret band compared with that of the neutral species, which shows a single
maximum. The characteristic band in the 600 nm region disappears in the

spectrum of the cation radical and is replaced by several weak, broad bands

93

 

 

 

 

 

 

 

WAVELENGTH (nm)

Figure 2. Optical absorption spectra of CuMeOEC(-) and its one-electron oxidation
product, CuMeOEC““C104‘ (-—) in CHzClz.

94

that extend into the near infrared region. Figures 3 and 4 show the RR spectra
of t-CuOEC 1, and the Cu(II) 2, Co(II) 3, and Ni(II) 4 complexes of MeOEC and
their one-electron oxidation products, respectively. The methyl substituted
Cu chlorin complex, 2, exhibits a Raman spectrum that is nearly identical to
that of the corresponding Cu m-OEC complex, 1, in both its neutral and its
oxidized state. The only clear exceptions to this generalization are slight
. frequency shifts in lines that occur at 1508 and 1602 crn‘1 in 1 and the splitting
of the line that occurs at 1204 cm"1 in 2 into two features at 1198 and 1212 cm'1
in 1. We conclude that the added stability imparted by alkyl substitution of
the chlorin radicals may be gained without serious alterations to the
vibrational properties of the dihydro gags-octaethylchlorin macrocycle.

Relative to the Raman spectra of MP, those of MC are considerably

more complex owing to the symmetry reduction in proceeding from D4h in
the MP case to at least sz for the MC complexes, to increased macrocycle
conformational flexibility, and to the likelihood of changes in normal mode
composition of the chlorin macrocycle vibrations compared to those of the
analogous porphyrin.20,21r22 As a result, additional modes become Raman
allowed in the metallochlorins. This effect is evident in the spectra in Figures
3 and 4. For example, in the 1360-1390 cm'1 region of the neutral MC species
at least three modes are apparent; in the case of the corresponding

metalloporphyrins only a single mode (V4) is generally observed.23 Likewise,

in the higher frequency region AV > 1500 cm‘l), where macrocycle stretching
modes with CaCm and Cbe character occur, the MC spectra are more
complex. For example, Boldt g1a_l. have identified ten vibrations in the 1450-
1700 cm'1 region of the RR spectra of t-NiOEC (solid state) by using

excitation at various wavelengths.22 In the present study, we focus on two

95

 

c) CuMeOEC

>._._m2wh2H Z<§<m

 

 

:1) N1 MeOEC

 

I700

RAMAN SHIFT (cm")
Near-uv RR spectra of t-CuOEC (a), Cu 0)), Co (c), Ni (d) complexes of

MeOEC. The laser power was 20-30 mw. The solvent (CI-IzClz) band at

1423 curl is labeled with an asterisk (a).

Figure 3.

96

A” ' 363.8 nm

 

a) t- CuOEC’

c) CuMeOEC

C6558” zazdm

 

d) Ni MeOEC?

  

' 1300' '

RAMAN SHIFT Ian")
R spectra of (a) t-CuOEC"’°, (b) CuMeOEC“, (c) CoMeOEC‘", (d)

N iMeOECT'.

 

Figure 4.

The laser power was 20-30 mw. The solvent (CHzClz) band

at 1423 czn‘1 is labeled with an asterisk (‘).

97

modes in the 1500-1520 cm'1 and 1620-1650 cm“1 region that have analogy to
those that are referred to as V3 and V10 vibrations, respectively, in the
porphyrin analogs. These modes are clearly defined in all species and exhibit a
strong core-size dependency in the porphyrin complexes.

We have applied the empirical correlation first reported by Spaulding

_e_t_ a_1. between the Raman frequency and the macrocycle core-size to the V10-

and V3- like vibrations of the chlorins.24 The parameters K(cm'1 / A) and

A(A) in the relationship, AV = K(A-d), have been estimated by making the
following assumptions. First, the core size (d,A) remains constant upon
oxidation of the macrocycle in the chlorin complexes, as we have found for
the metalloporphyrin systems15 and second, we may use the core-size
distances already established for the porphyrin counterparts.24 Based on
limited x-ray crystal data on the chlorin system, however, the latter
assumption may not be fully accurate.25

Table 1 summarizes the pattern of frequency shifts observed for the
V10- and V3-like vibrations, as well as the optical absorption shifts of the Soret
band maxima, associated with both porphyrin and chlorin metal complexes
when they are oxidized at the ring. Table 2 lists the K and A values for these
vibrations. A number of observations concerning the v10 and V3 vibrations of
the MC/MC+' and MP/MP+° complexes require comment. Because the
relatively high K values for the V10 modes of both systems are similar (- 400
cm'l/A), we conclude that these vibrations are predominantly CaCm
character in both cases. On the other hand, for the V3 mode, we see that
the K values for the MP/MP+' system (~ 380 cm'1/A/) are significantly
higher than those of Mc/MC+° system (~ 300 cm'l/ A). This implies that the

percentage

98

C aCm character in chlorin vibration is correspondingly lower than in the
porphyrin V3 vibration. In MOEP, the V3(CaCm) vibration has no Cbe
character. However, the V3-like chlorin vibration around ~ 1500 cm‘1 may

have substantial Cbe character. The contribution of Cbe character to this

mode in the chlorins is implied by the lower K value and may also be realized
by comparing its frequency in copper cis-dihydroxyoctaethyl chlorin21 at 1500
cm‘1 with those of 1 and 2 at 1508 and 1506 cm‘1 in the neutral species and at
1512 and 1509 cm‘1 in the cation radicals, respectively. The frequency

variation with the change in peripheral substituents on the chlorin ring

implies a contribution from the Cbe stretching coordinate to this normal

mode.26 Thus, attaching the V3 label to this chlorin macrocycle vibration

does not imply that the normal mode composition is the same as in the
porphyrin case. For this mode, at least, our analysis supports that of Boldt g1
a_l.22 who argued that the normal coordinates in the metallochlorins may be
significantly different than those in metalloporphyrins.

In previous work, we found that a general pattern of frequency shifts

occurred in the MP1" relative to their neutral MP precursors. Modes that

have significant Cbe character were found to increase; whereas, those that

are principally CaCm and CaN in character decrease relative to the neutral

metalloporphyrin.15 We also concluded that there was little expansion or
contraction of the macrocycle core upon oxidation and that the normal mode
composition in the neutral precursor was largely preserved in the lt-cation
radicals, at least for those high-frequency modes. Specific deuteration
experiments on the MP and MP1" species provide support for these

conclusions.27

99

The data in Table 2 indicate that the K values for the V3 and V10

vibrations in both the MP and MC systems do not change significantly upon
oxidation to the corresponding MP4" and MC“ system. This implies that the
PED of these chlorin vibrations also remains relatively constant upon
_ oxidation. Table 1 reveals, however, that the empirical observations made
concerning frequency change due to oxidation for the V3 and V10 modes in the
MP / MP1" system do not apply directly to the corresponding MC/MC+°
vibrations. In the chlorin case, the V3 frequency increases upon oxidation.

We consider this as further evidence for Cbe stretching contribution to this

mode, as the small increase (~ 3 cm‘l) in the frequency of this mode upon

oxidation in 1-4 may be the result of offsetting effects. In the

metalloporphyrin case, Cbe character causes a strong frequency increase,
whereas CaCm stretching coordinate contributions cause a weak frequency
decrease. Thus, the result of mostly CaCm and small Cbe character is a slight

frequency increase upon oxidation. We also note that the change in V10

frequency that occurs upon oxidation, though qualitatively similar, is perhaps
larger in the chlorin than in the porphyrin case.

In the 1540-1620 cm'1 region of the CuMeOEC+' spectrum, we
recognize two modes at 1604 and 1612 cm'1 that appear to have substantial
Cbe stretching character. These modes have similarities to the V11 and V2
vibrations that occur in CuOEP+' at 1600 and 1610 cm‘1, respectively. Further
confirmation and resolution of the modes in this region will be provided by
selective isotopic labelling, core-size plots, IR, and both visible and Soret

excitation RR measurements, which are in progress.

100

Two points follow from the data presented in this study.

The alkyl-substituted MMeOEC complex serves as a suitable system for
the resonance Raman characterization of its ring-centered oxidized
species; serious alteration of the vibrational properties of the chlorin
macrocycle in this species, as compared to those of t-OEC system, does not
occur.

The preliminary core-size plots as well as the pattern of frequency shifts
observed for the V3 and V10 vibrations upon oxidation reveal similarity of
the chlorin and porphyrin complexes; however, differences in the normal
mode composition and the number of Raman allowed vibrational modes
between the two systems will provide an additional source of complexity

in the spectral analysis of the metallochlorin species.

101

Table I

Resonance Raman Frequencies (cm'l) and
Optical Absorption Maxima (nm) for
Parent MOEP and MMeOEC and Their

Corresponding n-Cation Radicals

Compound V3 V3+° AV3OX V10 V10+° AV?8( Soret,nm

Neutral Oxidized

 

Mi OEP 1519 1511 -8 1655 1651 -4 391 377
MeOEC 1514 1520 -6 1652 1640 -12 399 368, 391
CoOEP 1512 1505 -7 1647 1642 -5 391 376
MeOEC 1512 1514 -2 1650 -1639 -11 392 368, 382
Cu OEP 1502 1497 -5 1636 1631 -5 397 383
MeOEC 1506 1509 -3 1642 1632 -10 399 374, 389
21101313 1485 1477 -8 1618 1617 -1 400 387
MeOECb 1490 1490 0 1619 1608 -11 400 381, 392

 

 

 

Abbreviations: V3+', the V3 frequency (cm-1) of the cation radical: AV30x frequency shift
(cm'l) for the V3 vibration due to one-electron oxidation (AV3OX - v3+- -

V3): the preceding descriptions also apply to V101" and Avolg,

respectively.

a porphyrin data taken from: Oertling e_t a_l. (J. Phys. Chem. 1987, 21, 5887 - 5898): chlorin
data from this work.

b ZnMeOEC was oxidized by a dilute methanolic solution of iodine in CHZCIZ (RR spectra not
shown).

102

Table 11

Estimated K(cm‘1/A/) and A(A) Values for
the V and V Vibrations of the Parent MOEP,
MMeOEC and Their it-cation Radicals

Parameters V3 v31" v10 V3

 

MOEP, K 383 384 415 377
A 5.93 5.90 5.94 6.33
MMeOEC, K 278 334 385 375
A 7.41 6.51 6.26 6.34
_ _ —

 

 

Abbreviations: V3+‘ and V31" , the V3 and V10 frequencies (cm‘l) of the cation radical,

respectively.

10.

11.

12.

REFERENCES

Mathies, P.; Rutherford, A. W. In "Photosynthesis," Amesz, I. Ed.,
Elsevier: Amsterdam, 1987, Q.

Ranger, G., Angew. Chem. Int. Ed. Engl., 1987, 2_6, 643.

Davis, M. S.; Forman, A.; Fajer, 1., Proc. Nat. Acad. Sci. (USA), 1979, 26,
4170.

O'Malley, P. J.; Babcock, T. G., Proc. Nat. Acad. Sci. (USA) 1984, 81, 1098.

Hoff, A. J., In "Antennas and Reaction Centers of Photosynthetic
Bacteria", Michal-Beyerle, M. E., Ed., Springer: Berlin, 1985, 150.

Lutz, M., In "Advances in Infrared and Raman Spectrosc0py", Clark, R. I.
and Hester, R. e., Ed., Wiley Heyden: New York, 1984, Vol. 11, 211.

Fuhrhop, I. -H., Z. Naturforsch, 1970, 2%, 255.

Chang, C. K.; Hanson, L. K.; Richardson, P. E; Young, R.; Fajer, 1., Pre. Nat.
Acad. Sci. (USA), 1981, 7_8, 2653.

Chang, C. K., In ”The Biological Chemistry of Iron", Dunford, H. B., Ed.,
Reidel, D.: Philadelphia, 1982, 313.

Hanson, L. K.; Chang, C. K.; Davis, M. S.; Fajer, J., I. Am. Chem. Soc., 1981,
m3, 663.

Fujita, E.; Chang, C. K.; Fajer, I. Am. Chem. Soc., 1985, l_(_)_Z, 7665.
Stolzenberg, A. M.; Stershic, M. T., Inorg. Chem., 1987, 26, 1790.

103

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

26.

27.

104
Scheer, H.; Inhoffen, H. H., In "The Porphyrins", Dolphin, D., Ed.,
Academic Press: New York, 1978, Vol. H, 45.
Iacob, G. 5.; Orme-Iohnson, W. H., Biochemistry, 1979, 13, 2967.

Oertling, W. A.; Salehi, A.; Chung, Y. C.; Leroi, G. E.; Chang, C. K.;
Babcock, G. T., I. Phys. Chem., 1987, fl, 5887.

Salehi, A.; Oertling, W. A.; Babcock, G. T.; Chang, C. K., Inorg. Chem.,
1987, g, 4296.

Whitlock, Ir., H. W.; Hanauer, R.; Oester, M. Y.; Bower, G. K., I. Am.
Chem. Soc., 1969, 21, 7485.

Chang, C. K., Biochemistry 1980, 12, 1971.

Salehi, A.; Oertling, W. A.; Babcock, G. T.; Chang, C. K., I. Am. Chem. Soc.,
1986, 1Q; 5630.

(a) Ozaki, Y.; Iriyama, K.; Ogoshi, H.; Ochia, T.; Kitagawa, T., I. Phys.
Chem. 1986, fl), 6105.
(b) Ozaki, Y., Iriyama, K.; Ogoshi, H., Kitagawa, T., I. Phys. Chem., 1986,
9_0, 6113.

Andersson, L. A.; Sotiriou, C.; Chang, C. K.; Loehr, T. M., I. Am. Chem.
Soc., 1987, 1%, 258.

Boldt, N. I.; Donohoe, R. I.; Birge, R. R.; Bocian, D., I. Am. Chem. Soc.,
1987, m, 2284.

Abe, M.; Kitagawa, t.; Kyogoku, Y., I. Chem. Phys., 1978, Q2, 4526.

Spaulding, L. C.; Chang, C. C.; Yu, N. -T.; Felton, R. H., I. Am. Chem. Soc.,
1975, 9_7, 2517.

Strauss, 5. H.; Silver, M. E.; Ibers, I. A., I. Am. Chem. Soc., 1983, fl, 4108.
Callahan, P. M.; Babcock, G. T., Biochemistry, 1981, _2_0, 952.

Oertling, W. A.; Salehi, A.; Chang, C. K.; Babcock, G. T., I. Phys. Chem.,
submitted.

105

Supplementary Material: In order to facilitate a detailed investigation of
vibrational mode assignments of the chlorin ring macrocycle at both the
neutral and n-cation radical levels, selectively deuterated trans-
octaethylchlorin derivatives have been prepared which will be addressed in
the following section; the normal mode assignments, however, will be the
subject of a future work and will be presented elsewhere by other coworkers

(Harold N. Fonda and Anthony W. Oertling).

Preparation of trans-octaethylchlorin-d2(a,B),-d2(y,8), and -d4 as well as
octaethylporphyrin-d2 using acid-catalyzed exchange reaction

Deuteration of the methine (meso) bridges of tetrapyrrolic macrocycles
has contributed to a better understanding of the hemoproteins' electronic and
vibrational characteristics by utilizing methods such as ESR(1,2), EN DOR (3,4),
NMR (5,6), IR(7,8) and resonance Raman (9,10) spectroscopy. The relative
reactivities of the four methine carbons for hydrogen-deuterium (H/ D)
exchange result primarily from the ease of electrophilic substitution at these
sites which, in turn, depends upon the nature of the substituents on the
adjacent pyrrole rings (11,12,13). For example, it has been shown that trans-
octaethylchlorin (t-OEC) undergoes facile H/ D exchange at 7,8-meso carbons
in refluxing acetic acid-d1; whereas, a,B-bridges are not affected (14). The
exchange reactions, carried out on metal free systems, are generally acid-
catalyzed and a number of acidic media have been reported, including D2504-
D20 (11), toluenesulfonic acid-d1 (15), TFA-d1 (12), and acetic acid-d1 (16).
The D2504-D20 method represents a strong acidic condition which is

routinely used to induce H/ D exchange reaction at all four methine carbons

106

in symmetrically substituted ring systems (12). In one instance, however, it
has been applied to the oxoporphyrin complexes in which deuteration occurs
predominantly at two bridging carbons and the other two sites undergo
minimal exchange (17). The D2504-DzO method is usually carried out at
ambient temperature in contrast with the refluxing and/ or anaerobic
condition required by other methods.

We report here a modification of the original D2504-DzO reaction
condition (11) which may be used for the preparation of three forms of
selectively deuterated trans-octaethylchlorin(t-OEC-d2(a,B),-d2(y,6), and -d4),
Figure 5a. The dideuterated chlorin species may then be oxidized to the
corresponding octaethylporphyrin-d2, Figure 5b, which would otherwise be

difficult to prepare and may reveal interesting spectroscopic properties.

 

(a) t-OEC (b) OEP

Figure 5. Chemical Structures

107

EXPERIMENTAL

OEP and t-OEC were synthesized according to the published methods
(18, 19). Proton NMR spectra were obtained at 250 MHz on a Bruker
spectrometer in deuterated chloroform with the solvent signal as an internal
standard. Chloroform-d1 (99.8 atom percent D), D20 (99.8 atom percent D)
and 98 percent sulfuric acid-d2 in D20 (99.5 atom percent D) were obtained
from Aldrich Chemical Co. Dichloromethane was freshly distilled from
CaHz. All other solvents were reagent grade and were used without further
purification. The extent of deuteration was estimated by integration of the
proton NMR spectra. Column chromatography was performed on Alcoa

basic alumina (type F-20).

trans-octaethylchlorin-d2(y,5)

In a typical experiment, 10 mg of t-OEC was equilibrated in 2.1 ml of
D2504zDzO (6:1,v/v) and allowed to sit at room temperature in a desiccator
for 20 hours. The solution was then quenched with ice water and extracted
with dichloromethane. The extract was washed with sodium bicarbonate and
with water and dried over anhydrous sodium sulfate. At this point, the
sample may contain about 2-3 percent OEP which results from oxidation of t-
OEC. This was demonstrated by t.l.c. and by proton NMR spectrum (OEP
meso-proton peak at 8 10.2). Purification was carried out on alumina with
benzene as eluant. Quantitative deuteration was established by the
disappearance of the 7,8- proton signal at 8.9 ppm, Figure 6c. M5: m/ e
538(M+), 509(M+-29), 269(M++), Figure 7b; mp. 231-232°C.

Figure 6.

.0 I'OIC

108

 

 

 

 

(_
Ff

 

 

 

 

fiTTT'TfT T'T'I'T'T'I'WTTTI'I

10.0 0.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -I.0 -2.0 -3.0

PPM

ZSO-MHZ proton NMR spectra of t-OEC and its deuterium labeled
derivatives in CD03. 5 indicates residual undeuteriated solvent and X

represents other impurities.

109

So an 3 8333.8“. 9.25% a:
pad 582035080932. 3 32-—m. 3.00% and... EEE. sebum—o 05,—. K 2:9"—

 

 

 

 

 

 

 

 

 

 

 

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109

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6:0 532035020938. Co 32-—m. 2.3% «was. 82?: .8300.» 2:. N 9.sz

 

 

 

 

 

 

 

 

a“ 'F" DDDDDDDDD ‘H O DDDDDDDDD q. bbbbbbbbb d DDIbDDDDD Flip} I‘D’FNDhPDLFDD bbbbbbb 3- .hbbbbhp)>.l-.-1DI<PPII I.>.or t d.‘
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. u. .2. a. +4.. .1... 9
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110

trans-octaethylchlorin-d4

The tetradeuterated species was prepared by the same method as
described above; however, the sample was permitted to sit at room
temperature for ~ 72 hours. One hundred percent deuteration was
established by the disappearance of proton NMR signals at 8.9 and 9.7 ppm
due to 7,8- and a,B- protons, respectively, Figure 6d. M5: m/ e 540(M+), 511
(M‘H'), Figure 7c.

trans-octaethylchlorin-dz(a,B)

t-OEC-d4 (10 mg) was subject to "back-exchange" in 2.1 ml of
H2504(96.4 percent):H20(6:l, v/v) followed by the same work-up as
mentioned above. This method gave ~ 90 percent deuteration at 01,8-

position and complete proton recovery at the 7,5- sites, Figure 6b.

octaethylporphyrin-d2

To a refluxing dichloromethane solution of dideuterochlorin was
added a solution of DDQ (excess) in benzene according to the published
methods (20,21). The green color of the chlorin solution turned red almost
instantaneously. After refluxing for an additional 0.5 h, the solution was
evaporated to dryness and chromatographed on alumina with chloroform.
The visible absorption spectrum and the proton N MR signal at 8 10.2 showed
the characteristic OEP spectrum. M5: m/e 536(M+), 507(M+-29), and
268(M++); mp. 322-324°C.

111

CONCLUSION

The original procedure reported for the tetradeuteration of OEP (80 mg
in D2504zDzO, 2 ml; 9:1, w/v) has been modified to prepare three forms of
selectively deuterated trans-octaethylchlorin; namely, -d2(a,B),-d2(y,5), and -
d4 derivatives. This method offers an alternative approach to that of acetic
acid-d1 for H/ D exchange at ambient temperature in unsymmetrically
substituted tetrapyrrolic macrocycles. It is also particularly attractive for
deuterium labeling in systems where prolonged refluxing conditions are not
desirable. The partially-deuterated porphyrin and chlorin complexes are
currently being utilized to verify the proposed "quadrant-localized"
vibrational modes in the chlorin macrocycleszz, by means of IR and

resonance Raman spectroscopy.

10.

11.

12.

13.

REFERENCES

Fajer, I., Borg, D.C., Forman, A., Dolphin, D., I. Am. Chem. Soc.,1970, 42:
3451.

., Mengersen, C., Subramanian, I., Fuhrhop, I.W., Smith, K. M., Z.

N aturforsch, 1974, gag 1827.

Norris, I. R., Scheer, H., Druyan, M.E., Katz, I.I., Natl. Acad. Sci. USA, 1974,
_7_1: 4897.

Feher, G., Hoff, A.I., Isaacson, RA., Ackerson, L.C., Ann. N. Y. Acad. Sci.,
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Morishima, I., Takamuki, Y., Shiro, Y., I. Am. Chem. Soc., 1984, 126: 7666.
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Smith, K. M., Langry, K. C., deRopp, I. 5., I. Chem. Soc. Chem. Comm.,
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112

14.

15.

16.

17.

18.

19.

20.

21.

113

Cavaleiro, I. A. 5., Smith, K. M., I. Chem. Soc., Perkin Trans. 1973, 1, 2149.
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Stolzenberg, A. M., Laliberte, M. A., I. Org. Chem., 1987, 5_2: 1022.

Wang, C. B., Chang, C. K., Synthesis, 1979, 548.

Whitlock, Ir., H. W., Hanauer, R., Oester, M. Y., Bower, B. K., I. Am.
Chem. Soc., 1969, 21: 7485.

Eisner, V., Linstead, R P., I., 1955, 3749.
Barnett, G. H., Hudson, M. F., Smith, K. M., Tet. Lett., 1973, 2887.

Boldt, N. I., Donohoe, R. I., Birge, R. R., Bocian, D., I. Am. Chem. Soc.,
1987,fl2: 2284.

CHAPTER V

NOVEL B—PYRROLIC SUBSTITUTION REACTIONS OF
TETRAPERFLUOROPHENYLPORPHYRIN

Tetraperfluorophenylporphyrin (TFPP) is an electron-deficient
porphyrin having highly electron-withdrawing yet chemically inert meso-
perfluorophenyl substituents. This pattern of peripheral substitution is
expected to raise the redox potentials of the porphyrin macrocycle
dramatically. For example, half-wave potentials of TFPPFeCl are positively
shifted by 0.4 and 0.32 V with respect to those of TPPFeCl for the first and
second oxidation waves, respectively.1 On the other hand, flourine
substitution, particularly on phenyl C-2 and C-6 positions, provide the
porphyrin ring with added steric protection against oxidative attack which is
commonly directed at the meso carbons. This reaction often results in the
cleavage of the ring and formation of an open-chain tetrapyrrolic structure.2
This type of electrophilic substitution is operative in biological heme
catabolism and it has also been demonstrated with the OEP and TPP model
compounds.3 Thus, increased stability against self-destruction as well as
higher oxidation potentials make the TFPP system a powerful oxidation
catalyst for the study of cytochrome P-450 mediated reactions.4 For example,
Chang and Ebina have shown that the TFPPFeCl-catalyzed hydroxylation of

cyclohexane to cyclohexanol proceeds in 71% yield compared with the 5%

114

115

yield obtained from TPPFeCl.5 Apparently, the electron-deficient TFPP com-
plex forms a' more reactive and electrophilic oxene complex (TFPP+°FeIV=0).

During a chemical and spectroscopic investigation of the metallo TFPP
complexes, Gouterman and coworkers observed a new series of TFPP adducts.
These adducts were formed when the free base was treated with AgNO3 in
acetic acid or, alternatively, the corresponding metal derivatives (e.g., Ag2+,
N221; Zn2+ and Cu2+) were dissolved in acetic acid in the presence of
catalytic amount of concentrated nitric acid, under refluxing condition. A

This work aims to elucidate the structures of these TFPP, derivatives by

Uv-vis, FAB-M5 and lH-NMR studies.

General Characteristics of TFPP
a. Reduced Affinity to Metal Ions: The insertion of Coz+ ion, for

example, into HzTPP may be carried out by refluxing a chloroform-methanol

solution of the free base containing cobaltous chloride (or acetate), while

under identical condition, most of H2TFPP remains as a free base even after

24 hours. Thus, CoTFPP may be prepared by refluxing an acetic acid solution
of H2TFPP which contains a large molar excess (10x) of cobaltous acetate.

b. Resistance Toward Acid Demetallation: N iTFPP, for example, resists
demetallation considerably. N iTPP can be completely demetallated with

H2504 at RT, while a super acid (e.g., FSO3H) is required for the TFPP

derivative. Gouterman e_t 81. reported an alternative method by refluxing a
trifluoro acetic acid solution of N iTFPP for ~ 72 hours.14

c. Reduced Proton Affinity: H2TFPP exists as free base in acetic acid. In
a marked contrast, HZOEP exists as a monocation in acetic acid.6 The great

dicationic salt of H2TFPP may readily be formed by the addition of TFA,

116

HN O3 and HClO4 to a dichloromethane solution of the free base. Table I

summarizes the optical changes associated with the formation of

H4TFPP2+2X'.

Table I. Absorption Spectra (nm) of Dicationic Salt of H2TFPP in CHzClz.

 

 

 

 

Compound Soret IV III II I
H2TFPP 412 506 - 583 655
H4TFPP2+°2X'

X' = TFA‘ 430 538 574 625
N 03' 434 543 579 629
ClO4' 433 ? 580 631

 

The preceding observations as well as the increase in oxidation
potentials of TFPPFeCl by ~ 0.4V with respect to those of TPPFeCl demon-
strate that introduction of the fluoro groups on the phenyl rings results in a
significant reduction in the electron density on pyrrolic nitrogens.

d. Optical Absorption Properties: TFPP complexes feature unusual
electronic spectra which separate them from other tetraphenyl porphyrins.7
The energies of absorption maxima in the free base and the metal derivatives
fall between those of OEP at higher energy and TPP at lower energy, Table 2.

The most remarkable difference is observed in the spectrum of H2TFPP in

which an absorption maximum near 550 nm (band 111) is absent, in contrast

with that of HZTPP.7

117

Table 2. Comparison of H2TFPP Absorption Maxima (nm) in CH2C12 with
Those of HZTPP and HZOEP.

 

Compound Soret IV III II I

 

H2TFPP 412 506 - 583 655
HZOEP 397 496 531 565 615

 

Kim and coworkers later confirmed the phenomenological cause of this

anomalous behavior.8 They found that electron-withdrawing substituents in
the phenyl ortho position cause the diminishing of the extinction coefficients
of bands 1, HI, and Soret; whereas, bands II and IV are not affected. Moreover,
phenyl substitution in other positions have minimal effect.

The visible absorption spectra of TFPP complexes exhibit strong
solvent-dependent maxima shifts, Table 3. Thus, if optical changes, which are
accompanied by TFPP substitution reactions, are to be used to partially
characterize the newly-formed adducts, the solvent-induced shifts must also

be taken into consideration.

118

Table 3. Solvent-induced Absorption Maxima Shifts (nm) of TFPP Adducts.

 

Compound Solvent Soret IV 111 II I

H2TFPP DMF 410 504 — 579 654
CH2C12 412 506 - 583 655
0H 417 508 - 586 659
¢CH3 417 509 - 586 658

N iTFPP CH2C12 404 524, 558 620
¢CH3 407 526, 559 621
PY 429 553 621

CuTFPP CHzClz 408 534, 570
¢CH3 416 539, 573

ZnTFPP CHzClz 413 543, 577 606
¢CH3 421 547, 580 607

 

e. 1H-NMR: H2TFPP and its N12+ and th+ complexes feature a
single peak at 8.50, 8.77 and 8.99 ppm, respectively, in CDC13 characteristic of
B-pyrrole protons. The upfield shifts of the B-hydrogen peaks in the TFPP
adducts as compared with those observed in the TPP system (e.g., HZTPP, H3
= 8.75 ppm) has been explained in terms of the degree of buckling of the
porphyrin nucleus due to meso-fluorophenyl substituents.9 Apparently, the
steric interaction between the fluoro groups and the B-hydrogens of the
pyrrole rings results in increased c0planarity of the phenyl groups with the
porphyrin ring in the TFPP system. This, in turn, reduces buckling of the

macrocycle and increases the ring current effect.

119

f. Reversible Oxidation Reactions of MTFPP Derivatives - Electronic
Absorption Characteristics: Ferric perchlorate salt, Fe(ClO4)3°XHzO, serves as
a chemical oxidant of general utility and provides adequate redox potential
for at least single-electron oxidation of all common porphyrins (81 /2 = 1.2V vs.
SCE). The utility of this oxidant has been demonstrated in several instances
in the previous chapters. However, it falls short of oxidizing Cu2+ and N 12+
complexes of TFPP. The Zn2+ adduct demetallates immediately upon
oxidation. This may result from 1) large ionic radius of Zn2+ ion, 2) electron-
deficient porphyrine ring and 3) further weakening of the metal-porphyrin
bonds upon removal of an additional electron from the porphyrin )t-system
to form a cation radical.

However, the Ag2+ and Co2+ complexes of this porphyrin are readily
oxidized at the metal to form the corresponding valance +3 derivatives. The
absorption-shift pattern observed for both systems upon oxidation agrees well

with the Ag(III) and Co(III) complexes of other porphyrins, Table 4.10: 11

Table 4. Changes in the Optical Absorption (nm) in CHzClz of the AgII and
CoII Complexes of TFPP Upon One-Electron Metal-Centered Oxidation.

 

 

 

 

Compound Soret Visible Bands
AgHTFPP 419 537, 570
AngFPPClO4' 418 527, 559
CoHTFPP 404 527
ComTFPP(HzO)2ClO4‘ 422 536

w m

 

120

Our attempt to chemically oxidize CoIITFPP by more than one electron
resulted in the partial formation of CoIIITFPP+°2ClO4' (e.g., 666 nm) and that
the CoIII species (e.g., 422 and 536 nm) persisted for the most part. The
presence of the radical was conformed by observing a broad, residual ESR
signal at room temperature. Qualitatively, it is expected that the TFPP
radicals are more prone than their TPP counterparts to adopt 2Aw electronic
ground state or that the fluorophenyl groups may completely stabilize the a2U
orbital relative to the an} orbital by withdrawing electron density from the
meso carbons.12 The ZAIU vs. 2A2U state of the MTFPP radicals may be
crucial for selective peripheral functionalization of the porphyrin ring either
at the B-pyrrolic or on the bridging meso-carbon positions, as was recently

reported by Catalano and coworkers for the analogous TPP complexes.13
EXPERIMENTAL

The reaction conditions applied in this study may be divided into three
categories as follows:

1. The free base H2TFPP was treated in glacial acetic acid with a large excess
(e.g., 10x) of metal nitrate salt (e.g., Ag2+, Cu2+) under refluxing condition
for ~ 6 hours.

2. The desired MTFPP (e.g., Ag2+, N121“) was initially prepared from
refluxing an acetic acid solution of the free base with a large molar excess
(e.g., 10x) of metal acetate. Subsequently, MTFPP was treated with acetic
acid in the presence of catalytic amount of concentrated nitric acid under
refluxing condition for ~ 6 hours.

3. The free base was refluxed in acetic acid in the presence of a large molar
excess of metal acetate (e.g., Zn2+) and catalytic amount of concentrated

nitric acid for ~ 6 hours.

121

In all above cases, removal of the solvent gave a green solid which was
redissolved in dichloromethane and chromatographed on silica gel TLC plate
(dichloromethane/hexane 9:1). The individual fractions in which their
optical spectra contained two bands in the 500-600 nm region and displayed
no fluorescence under the UV light were subsequently demetallated in TFA
under refluxing condition for 48 - 72 hours. The TFA solution was
evaporated to dryness and the solid was taken up in dichloromethane and
rechromatographed on silica gel (dichloromethane/ hexane 9:1) to afford the
free base adducts. This step, in some instances, resulted in the separation of
two or more fractions, which are not separable when they are metallated.
Moreover, demetallation would facilitate the 1H-N MR analysis of

paramagnetic species (e. g., Cu2+, Ag2+).

Results

Generally, three types of products were obtained based on their optical
absorption characteristics:

a. Type 1 features two bands of comparable intensity in the 500 - 600
nm region, characteristic of metalloporphyrins and designated as M(2-B).
Although Gouterman _t _1.7 have demonstrated the solvent-dependent
intensity variation of MTFPP visible bands, Figures 1 and 2, the so-called a
band intensifies drastically in M(2-B) species, Figure 3. This figure also
demonstrates that Cu(2—B) spectrum is red-shifted relative to CuTFPP. Type 1
species may be prepared by refluxing an acetic acid-nitric acid (catalytic, ~ 3
drops) solution of MTFPP for ~ 6 hours.

b. Type 2 species also displays a two-banded visible region; however, in

the absence of the central metal. This spectrum may arise from the mono-

122

 

 

 

Macaw-u

 

 

 

Wavelength (nm)

Figure 1. Absorption spectra of tetrakis(perfluorophenyl)porphyrin complexes
taken in CHzClz at room temperature: (A) free base; (B) Zn complex, (C)

Cu complex; (D) Pd complex. (From Ref. 7.)

123

 

 

Figure 2. Solvent-dependent absorption spectra of zinc tetrakis(perfluoro-
phenyl)porphyrin in visible region: (A) dry toluene: (B) toluene shaken
with HzO;(C) 75% toluene and 7.5% CHgCN; (D) 75% toluene and 25%
ethanol; (E) 75% toluene and 25% pyridine: (F) 75% toluene and 25%
triethylamine. (From Ref. 7.)

124

 

4|6

4O

ABSORPTION

 

 

 

\.___

 

 

-m

600 m“

 

 

Figure 3. Electronic absorption spectra of CuTFPP(-) and Cu(2-B) in CHzClz.

125

and diprotonated free base of the functionalized TFPP adduct, designated as
HM(2-B). In the absence of the exact identity of peripheral groups, type 2 may

be distinguished from type 1 by their active emission properties.” Type 2
adducts are formed as the by-products of M(NO3)2/HOAC or HOAC/HNO3

reaction conditions.
c. Type 3 adduct, designated as HM (4-B), shows a free base-like
spectrum in which band 111 is present, in contrast with that of H2TFPP. The

spectrum is also red-shifted relative to H2TFPP. The species derived from

AgTFPP features a band I which is more intense than bands 11 and III, Figure
4. Type 3 species may be prepared when M(2—B) is demetallated in TFA or, an
acetic acid-nitric acid (catalytic, 5 drops) solution of MT FPP is refluxed for ~6
hours.

The splitting of the exo B-pyrrolic protons in the 1H-NMR spectra of
the newly-formed TFPP adducts provides evidence for the functionalization
of B-pyrrolic double bonds. The extent of substitution is not clear in all cases;
however, resolution of nine or ten bonds in a few instances reveal that up to
two or three B-pyrrolic substituents may be present, Figure 5a and b.

In the FAB mass spectra of these adducts, observation of peaks at m/z
975 (M+, H2TFPP), 993 (M+, H2TFPP + H20), 1006 (M+, H2TFPP + NOH), 1020

(M+, H2TFPP + N02), 1038 (M+, H2TFPP + HNO3), as well as peaks alluding
to higher order addition of these substituents and their metal derivatives

shed more lights on the nature and extent of these substitution reactions.

Cu(II) or Ni (11) Adducts
CuTFPP in HOAC-HNO3 produced one major green fraction, which

was chromatographed on a silica gel plate with dichloromethane-hexane

(95:5) and collected as the least polar band. The absorption maxima of this Cu

ABSORPTION

126

 

 

 

 

 

 

 

 

 

4 I I
I
5 I I
I /\
545
J 1/ ‘AOQL’
400 500 600 nm

Figure 4. Absorption spectrum of H Ag(4‘3) in CHzClz.

127

 

 

 

 

 

 

 

 

 

 

 

 

 

 

,4 M K

cm

Figure 5a. Expanded B-pyrrolic region of 1H-NMR spectrum of Zn(Z-B), fraction 2,
in c003.

128

 

 

 

 

 

 

 

 

 

 

L l I l J l l I J
38 33 3.0 as ea
rl-m

Figure Sb. Expanded B-pyrrolic region of 1i-I-NMR spectrum of Zn(Z-B), fraction 4,
in CDC13.

129

(2-B) species, it demetallated derivative as well as those of CuTPP and its [3-
nitro substituted analogs are given in Table 5. This table shows a systematic
wavelength-shifts caused by the introduction of B-nitro group in TPP is

reproduced in the TFPP adducts. More importantly, a FAB-MS peak at m/z
1081 (M+, CuTFPP+NOz) confirms the assignment.

Table 5. Absorption Maxima (N m) of TFPP and TPP adducts in CH2C12.

 

 

Compound Soret Visible Bands
CuTFPP 408 533, 571
Cu(2-B) 416 543, 589
CuTPP 414 539
B-NOz-CuTPP 422 551
H2TFPP 412 506, 583, 655
ch(4-B) 419 518, 590, 646
HZTPP 417 514, 548, 592, 647
B-NOz-HZTPP 425 528, 561, 602, 664

 

NiTFPP reactivity is also analogous to that of Cu complex. One major
fraction was obtained under the same condition described earlier for the

copper derivative. Ni (2-B) species features absorption maxima at 423, 540,

and 592 nm and a FAB-V15 peak at m/z 1076 (M+, NiTFPP + N02).
Demetallation of Ni (2-B) in TFA resulted in the formation of HNi(4-B) with
absorption maxima at 424, 520, 595 and 650 nm. This spectrum, with the
exception of band I, is red-shifted relative to H2TFPP, analogous to that

observed in HCu (4-B).

130

Ag(II) Adducts
Demetallation of the reaction products of H2TFPP + AgNO3/HOAC in

TFA followed by chromatography on alumina resulted in the separation of
five green fractions. The absorption maxima and FAB-MS data are given in

Table 6.

Table 6. Absorption Maxima and FAB-Ms Data on AgTFPP Adducts.

 

 

Elution No. Amax (nm) m/z
1 422, 550, 595 1097 (M+,AgTFPP+HzO)
2 428, 562, 604 1038 (M+,H2TFPP+HNO3)
3 419, 515, 556, 594, 657 1038 (M+,H2TFPP4-HN O3)

993 (M+,H2TFPP+HZO)
4 422, 516, 555, 594, 649 1038 (M+,H2TFPP+I-INO3)

5 418, 510, 552, 595, 644 993 (M+,H2TFPP-l-HZO)

 

As it appears from the MS data in Table 6, addition of H20 and HNO3

moieties across the double bond and formation of chlorin-type structures is
evident. Although these fractions have been purified on alumina column,
they seem to be partially contaminated by one another. This is especially true
for fraction three in which there seems to be equal amounts of water and
nitric acid adducts present. If we rule out the possibility of functional group
transformation, 11;. (Irolysis, tautomerism15 ..... during the acquisition of FAB—

MS, then HPLC tel 'zmiques may be required to further purify these fractions.

131

Zn (II) Adducts

Reaction“ of ZnTFPP with HOAC/HNO3 resulted in the separation of five
major and a few minor fractions on silica plate after chromatography with
dichloromethane-hexane (5%) as eluant. Table 7 summarizes absorption
maxima and preliminary FAB-MS and 1H-NMR data for the major bands.
The proton NMR spectra of the Zn adducts, exhibit a more defined picture of
B-proton splittings than other derivatives and may be utilized as the subject
of further investigation. The prominent MS peaks of the fractions 1 and 2
indicate a higher order of substitution with Zn complexes than the previous
systems. The m/z at 1128 in fraction 1 corresponds to the addition of two
nitro groups and m/z at 1173 in fraction 2 corresponds to the substitution of

three nitro groups in the ZnTFPP complex.

Table 7. Absorption Maxima (nm), FAB-M5 m/z and 1H-NMR B-proton

Multiplicity Number (MN) of ZnTFPP Adducts.

 

1 436, 566, 613 1128 (MT) 2
(8.77 and 8.81 ppm)

2 435, 565, 610 1172 (MT) 9 or 10
1128 (M+-N02)

3 440, 572, 620 -- 3

(8,82, 8.86, 8.88 ppm)
4 439, 570-, 608 -- 9
5 436, 573, 619 -- multiple

_

132

Discussion

The original reaction condition used by Gouterman and coworkers, i.e.,
H2TFPP+AgNO3/HOAC was intended to find a more effective method for
silver ion insertion into H2TFPP. This is because the AgOAC/HOAC method
also gives poor yield. Indeed, the AgNO3/HOAC condition facilitates the
insertion of silver ion and metallation is quantitative in < 1 hour; however,
the reaction condition immediately proceeds to form the green adducts. The

FAB-M5 obtained on a sample which also refluxed for 3/ 4 hour already

indicates the presence of addition products across Cbe double bond, Scheme

1. It is possible to speculate that the electron-deficient TFPP system triggers a

H H H NOH H H H NO
OH H
H N/ N H N N ‘1 N /"
H H
+
M+=993 M =1007
H H
o H
NO H No2
/
L' N/ N H N N
+—
M+=1020 M -1038
Scheme 1

metal-catalyzed addition reaction to occur. Of the structures suggested, the
M+ = 993 and 1038 peaks are more prominent in this sample than the others,

Figure 6 a and b.

ABSORPTION

 

 

 

 

 

 

 

 

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"Gouterman reports AgTFPP 133,, at 416, 536 and 570 nm in 41013.

134

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To test whether this reaction is unique to the TFPP system, samples of
AgTPP and'AgTFPP were treated with HOAC/HN03 side-by-side. After
refluxing for ~ 4 hours, AgTPP remained intact whereas AgTFPP produced
the expected green products.

The electronegativity of the central metal may play a significant role in
the regiospecificity of addition, product selection and the extent of substi-
tution reaction.13 For example, Zn (II), Mg (II) and Cd (11) release electron
density to the porphyrin, thus making it more nucleophilic. This is perhaps
why higher order of substitution was observed with ZnTFPP. Ni (11), Cu (11)
and Fe (II), on the other hand, withdraw electron density from the porphyrin
ligand and, as a result, induce more electrophilic character in the ring.

The mechanism of aromatic nitration has been the subject of much
discussion in recent years.16 However, it is generally accepted that aromatic
cation radicals are considered as plausible intermediates in the mechanism

for aromatic nitration.17 A number of nitrating reagents have been applied

to the porphyrin system in the past decade, including Tl(NO3)3 or
CeIV(NO3)6 (NH2)2, 3N02/CH2C12 or (pH,13 Iz/CH3OH4-NaNOz,18
HOAC/HNO319 and NOz+BF4'/PY.20 Reaction of N02 with a MP+- of am
symmetry would lead to specific B-substitution, as was recently documented
by Catalano and coworkers for the nitration of Ni (II), Cu (II) and pd (II)
complexes of TPP. The mechanism of TFPP B-pyrrolic substitution reactions
however is expected to be more complex owing to the involvement of the
electron-deficient character of the TFPP system as a key factor, formation of
both hydration and nitration products resulting in chlorin and B-derivatized
porphyrin structures and possible participation of metals in the catalysis of

these reactions (0 g, Ag“). As was suggested earlier, it is tempting to

136

conclude that the metal complexes of the TFPP system are pro-2A“; cationic

radical state.‘ On the other hand, it may be that by virtue of steric protection of

meso carbons, the substitution reactit -.s are forced to occur at the B-pyrrolic

position despite the fact that the MTFPP+- may still be of ZAZU state. Thus, an

ENDOR measurement on CoHITFPP+°2ClO4' would be informative to

determine its ground state as ZAIU or 2AZU.

As it stands, this project is yet to be explored in several dimensions. A

few suggestions are as follows:

1.

ENDOR Measurements. This study would reveal that B-substitution
reactions of the TFPP complexes are directed by virtue of 1) the ZAIU
electronic ground state of MTFPP+', 2) steric protection of the meso
carbons by fluorine atoms which force B-pyrrolic substitution reactions to
take place or 3) the electronic distribution of the TFPP complexes is such
that the Cbcb double bonds reactivity approximates an isolated simple
alkene in which addition across the double bond proceeds easily. This
would then make it possible to test the reactivity of this system for other
classical electrophilic substitution reactions such as deuteration,
halogenation, formylation and acetylation.

High-field NMR spectroscopy, including decoupling and NOE difference
measurements would be invaluable to sort out the B-pyrrolic protons in
these newly-formed adducts. Moreover, the 19F NMR of these adducts
would offer an independent means of characterizing the substitution
reactions of the B-pyrrolic positions as well as the conformation of the
porphyrin ring. It has already been shown that in the 19F spectrum of

InTFPPCl, the spin-spin coupling patterns observed for the ortho and

137

meta fluorines of H2TFPP are doubled indicating the nonequivalence of

the two' sides of the phenyl ring. The signals for the para fluorine,
however, remained the same.8

Purification by HPLC Techniques: It has already been demonstrated by
Crossley and coworkers that methyl-nitro- and dinitro-adducts of the TPP

may only be purified satisfactorily by HPLC methods.21 This is especially
true for the AgTFPP adducts in which addition products from HZO,

HNO3 coexist in one fraction purified by plate chromatography.

10.

11.

12.

13.

14.

REFERENCES

Chang, C. K.; Ebina, F., Private Communication.

Cavaliero, I. A. 5.; Evans, B.; Smith, K. In "Porphyrin Chemistry
Advances," Longo, F. R., Ed., Ann Arbor Science: Ann Arbor, 1979,
Chapter 26, 335.

Evans, B.; Smith, K. M.; Cavaleiro, I. A. S. I.C.5. Perkin 1, 1978, 768.
Ostovic, D.; Knoblar, C. B.; Bruice, T. C., I. Am. Chem. Soc. 1987, 102, 3444.
Chang, C. K.; Ebina, F. I. Chem. Soc. Chem. Comm. 1981, 778.

(a) Stone, A.; Fleischer, E. B., I. Am. Chem. Soc. 1968, 9_0, 2735.
(b) Ogoshi, H.; Watanabe, H.; Yoshida, Z., Tetrahedron 1973, 22, 3241.

Spellane, P. I.; Gouterman, M.; Antipas, A.; Kim, 5.; Liu, Y. C., Inorg.
Chem. 1980, 386.

Kim, I. B.; Leonard, I. I.;.Longo, F. R., I. Am. Chem. Soc. 1972, 91, 3987.
Longo, F. R.; Finarelli, M. G.; Kim, I. B., I. Heterocycl. Chem. 1969, 6, 927.

Kadish, K.; Davis, D. G.; Fuhrhop, I. H., Angew. Chem. Int. Ed. 1972, 11,
1014.

Whitten, D. G.; Baker, E. W.; Corwin, A. H., I. Org. Chem. 1963, 2_8, 2363.

Fajer, I.; Davis, M. S. In "The Porphyrins," Dolphin, D., Ed., Academic
press: New York, 1979, Vol. IV, 197.

Catalano, M. M.; Crossley, M. I.; Harding, M. M.; King, L. G., I. Chem. Soc.
Chem. Comm. 1984, 1535.

Gouterman, M. Private Communication.

138

15.

16.

17.

18.

19.

20.

21.

139

Crossley, M. I.; Harding, M. M.; Sternhell, 5., I. Org. Chem. 1988, 63,, 1132.

(a) Politzer, P.; Iayasuriya, K.; Sjoberg, P.; Laurence, P. R., I. Am. Chem.
Soc. 1985197.. 1174.

(b) Ali, M.; Ridd, I. H., I. Chem. Soc., Perkin Trans. 2, 1986, 327.

Schmitt, R. I.; Buttrill Ir., 5. E.; Ross, D. 5., I. Am. Chem. Soc. 1984, 1%, 926.

Barnett, G. H.; Smith, K. M., I. Chem. Soc. Chem. Comm. 1974, 772.

Bonnett, R.; Stephenson, G. F., I. Org. Chem. 1965, 80, 2791.

Cavaleiro, I. A. 5.; Neves, M. G. P. M. 5., I. Chem. Soc., Perkin Trans. 2,
1986, 575.

Crossley, M. I.; Harding, M. M.; Sternhall, 5., I. Am. Chem. Soc.,
submitted.

CHAPTER VI

METAL-AXIAL LIGAN D VIBRATIONS IN METALLOPORPHYRIN S,
THEIR H—CATION RADICALS AND OTHER RELATED SYSTEMS

A PRELIMINARY VIBRATIONAL STUDY IN THE LOW-FREQUENCY
REGION (<1,ooo cm-l) ‘

In hemoproteins, their synthetic analogs and other related systems
such as vitamin B12, modulation of metal-axial ligand stretching frequency,
0(M-L), occurs as a function of macrocycle steric and electronic effects.1 The
frequency of this vibration is closely related to the nature of the M-L bond and
can provide additional information regarding the interactions between the
ring (cis-effect) and the extraplanar ligand (trans-effect) transmitted through
the central metal. Alternatively, this frequency can be used as a probe for the
metal coordination number, macrocyclic deformation and regiospecific
changes in peripheral substitution. Recent RR comparisons of the I)(V=O) in
various hemoprotein model compounds support this notion.2 Additionally,
several isolated reports of 0(Fe-C1) and 0(FeOFe) values measured by IR for a
series of ferric chloride and II-oxo dimer complexes have shown that these
frequencies differ as a function of the steric and electronic effects of a por-

phyrin peripheral substituents, Table 1.3

140

141

Table 1. Stretching Frequencies (cm‘l) of (Fe-Cl) and (FeOFE) Vibrations in
Several Different Ferric Porphyrin (P) Complexes.

 

 

(P) 0(Fe-Cl) £(FeOFe) Refs
Deut-DME 330 842 3a
TTP" 355 860 3b
T(MeO)3PP 378 865 3c
OEP 357 870 3d
TPP 365 876 3c
Proto—DME 880 3a
Meso—DME 885 3a
Acdeut-DME 886 3a
TPC 867 Be

   

sra(ylorprn

We have now begun to investigate the effect of porphyrin cation
radical formation on the RR metal-axial ligand stretching frequencies of a
series of ferric and cobaltic OEP radicals. This frequency may then be used to
determine metal coordination number in the oxidized porphyrin species.
Metal coordination may, in turn, affect ring stereochemistry and out-of-plane
deformations may influence vibrations below 1,000 cm‘l. The frequency
changes associated with M-L bonds (where M = Fe, Co and L = F, Cl, Br and I)
in PMm-L/ P+'Mm-L system, although of lesser biological relevance than the
PFe=O/ P+°Fe=O system, is helpful in evaluating the RR data that are
becoming available on the u(Fe=O) of ferryl porphyrins.3t4 Moreover, the M-
L bond sensitivity to the steric and electronic effects of porphyrin structure as
well as the effect of oxidized porphyrin on the metal-ligand CT transitions are
of interest.

A recent study by Halpern and coworkers on vitamin B12 .model

compounds suggest that the high degree of conformational flexibility of the

142

corrin ring serves an essential role in the chemistry of vitamin 312 as a

reversible "free-radical carrier".5 For this reason, the more rigid and planar
porphyrin ligand does not occur in this coenzyme. In connection with this
study, we present here the infrared 0(Co-C) of a series of alkyl-cobaltic
porphyrins of the type R-ComOEP (where R = Me, Et, i-Pr and t-Bu). These
stretching frequencies should reflect the changes in porphyrin ring conforma-

tion as the size of the alkyl groups increase.

EXPERIMENTAL

56FeOEPX, 54FeOEPX, CoOEPX and R-CoOEP were prepared according
to the published methods.6"9 One-electron oxidation products of the ferric
and cobaltic porphyrins were obtained as described in Chapters II and III.

Infrared spectra of R-CoOEP species were examined as KBR disks by.
using a Perkin-Elmer 599 IR spectrophotometer. Laser emission at 363.8 and
at 406.7 and 413.1 nm were provided by a Coherent Innova 100 Argon and
Innova 90 Krypton ion lasers, respectively. Raman spectra were measured

with a Spex 1877-B Triplemate and EG & G OMA III electronics.

Results 8: Discussion

Figure 1 shows the optical spectra of CoOEPX species. As described in
Chapter II, the characteristic electronic absorption spectra of five-coordinate
cobaltic porphyrins are quite distinct from those of six-coordinate derivatives.
A salient feature observed in this figure is the appearance of a shoulder on
the high-energy side of the band at ~ 550 nm which intensifies in the order Cl

<Br<I. This may partially be due to the more in-plane cobalt ion along this

series (i.e., decreasing EN/ri .=. electronegativity/ atomic radius) approaching

ABSORPTION

143

Conn) OEP):

XaCI 378

547
/
/ l
373
xaaf
54s
Jars
Xal
549

L

400 500 600 700 nm

Figure 1. UV-vis spectra of CoOEPX in CHzClz.

144

those of CoOEP and Com(L)20EPX which display active 0 bandsZ. The spectra
of ComOEP+°2X' for X .= C104 and Br are shown in Chapter II. When FeCl3 is

used as the oxidant, a cation radical spectrum is obtained, Figure 2, in which

the near-uv band at 634 nm lies in energy between those of X = Br at 670 nm

and X = C104 at ~ 620 nm. The overall spectral contour resembles more
closely that of the X = C104 adduct; however, the shoulders on the high-
energy side of the Soret band are a characteristic of X = Br spectrum. It is more
likely that X = FeC14 in this preparation because Setsune and coworkers

obtained a different spectrum when C12 was used as the oxidant.10

Figure 3 displays the optical spectra of ferric halide OEP complexes. The
fluoro complex features a shoulder at ~ 350 nm which is absent in other
derivatives. For X = F and C1, the visible region contains three distinct
absorptions; however, these bands are somewhat broadened out for X = Br
and I. The spectra of FemOEP+-2X’ show almost no sensitivity to the nature
of the counterion, in contrast with the neutral FeOEPX species as well as
ComOEP+'2X' complexes. A representative spectrum is shown in Figure 4.

The R spectra of CoOEPX, Figure 5, obtained at 363.8 nm are identical
except for a feature in the low frequency region at ~ 370 cm'1 that changes
systematically with X in the order of increasing energy: I<Br<C1. It is likely
that this feature is derived predominantly from in-plane vibration involving
the Co-N with possible contribution from the Co-X stretching motion, for the
following reasons:

1. The terminal MX stretching bonds have been shown to appear in the
regions of 750 - 500 cm-1 for MP, 400 - 200 arr1 for MCI, 300 - 200 cm-1 for
MBr and 200 - 100 cm‘1 for MI.18a

14S

 

 

 

398
2
o I
I:
Q
G
O
(D
m
<
5|8 577 634
L L I 1 L l l
400 500 600 nm

Figurez. Absorption spectrum of COmOEP'I'QX' (where X . FeCl4‘ or Cl") in
CHsz.

 

ABSORPTION

 

g r

146

FelmlCEPx

400 500 500 700 nm

Figure 3. UV-vis spectra of FeOEPX in CI-IzClz.

147

 

 

 

 

100 r , , I r T I
337 -
80? ~
1760
E I I
_U
I I-
2 ‘1
E
~40»- -
W
.. x2 ..
20-- 520 .1
A a a A L L gm 1 k a A A‘L L L A L
300 400 500 600 700

WAVELENGTH (nm)

Figure 4. Absorption spectrum of FemOEP+°(Cl')(SbC16‘) in CHzClz.

148

Co(III) OEPX
>‘ex' 363.8111?)

CH,cu

 

 

 

 

 

 

 

 

 

no») ll

0 ”a.

 

 

 

 

 

 

>._._mzm...ZH 2424”.

I600

|200 I400

FREQUENCY (cm")

I000

 

200 400 600 800

Figure 5. RR spectra of CoOEPX . The solvent bands

at 1422, 703 and 283 car1 are labeled with an asterisk.

149

2. According to Clark and Williams, the U(MBr)/0(MC1) and u(MI)/0(MC1)
ratios are'0.77 - 0.74 and 0.65, respectively.18b

3. Nakagawa and Shimanouchi have obtained the infrared spectra of
[Co(NH3)5X]2' and trans-[Co(NH3)4X2]+- type complexes.18C Table 2 lists
the observed frequencies and band assignments obtained by these
workers.

4. These workers also obtained the following force constants (mdyn/ A) for
Co-N and Co-X bonds: K(Co-N), 1.05; K(Co-F), 0.99; K(Co-Cl), 0.91; K(Co-
Br), 1.03,- and K(Co-I), 0.62.18d

5. In general, the infrared intensity of 0(MX) decreases in the order
0(MF)>1)(MC1)>1)(MBr)>u(MI), whereas the opposite order prevails for the
Raman intensity.18C ,

Basically, there is very limited information available on the vibrational
characteristics of five-coordinate cobaltic porphyrin complexes in the litera-
ture. Beside the fact that 59Co natural abundance is nearly 100% and halide
isotopic substitution is extremely difficult, Gouterman and Zerner have
shown that the cobaltic CT transitions are mostly of (1r,d22) and (ddeZ)
characterllrlz, in contrast with the ferric halide species in which X —) M CT
transition in the near-UV region (i.e., 350 - 370 nm) is predominant.13 Thus,
the RR spectra of C015 N-OEPX, Co-X derivatives of porphyrins other than
OEP and IR studies of these systems will be necessary to assign this mode
positively.

Figure 6 depicts that FeOEPX Raman Spectra, excited at 363.8 nm in
CHZCIZ, are also identical, however they do exhibit strong differences in the
relative RR band intensities owing to the differences in the near-UV

electronic transitions. The most striking difference is observed in the relative

150

>4” - 363.8 nm

 

 

 

 

 

snap-

 

 

 

 

 

 

snoutlll

Fe(III)OEPX

>._._mzm._.ZH

Z<2<m

 

 

l200 I400 l600

I000
FREQUENCY (cm")

 

200 400 600 800

RR spectra of FeOEPX .

Figure 6.

150

 

 

 

|000 (200 (400 (600

 

 

 

 

 

 

 

 

 

m
n
8.
3
6
3
.
x
e
\A
uncut II
OnONIII
x
P
E
MN 2
nu c
. u H:
.n. C

>._._.wszZH 24.24;.

 

 

 

 

200 400 600 800

FREQUENCY (cm")

RR spectra of FeOEPX .

Figure 6.

151

intensities of high and low frequency modes. A prominent feature is
observed at 574 cm'1 in the fluoro adduct which is absent in all other
derivatives. 54/56Fe frequency shifts (3 cm‘l) in CHzClz, Figure 7a, confirms
the 574 cm'1 band assignment as Fe-F stretching vibration. This is possibly in
contradiction to the previous assignment of 0(Fe-F) at 606 cm"1 in THF by
Kitagawa and Kincaid”:15 The discrepancy, on the other hand, may arise
from solvent effects, which are currently under investigation.

13d

Interestingly, the well-studied 0(Fe-Cl) at ~ 360 cm- is not observed

with excitation at 363.8 nm and in CH2C12, Figure 6. Figures 7b and 8,
however, reveal the presence of this mode at 360 cm-1 with excitation at 406.7
and 413.1 nm in 0H. The 0(Fe—F), on the other hand, is not enhanced under
this condition or it is obscured by the 6H peak at 606 crn-l, Figure 8. These
observations lead to the notion that 0(Fe-X) are not only CT enhanced, as
demonstrated by Hendrickson __t_ 81.13, but solvents may shift both these
electronic transitions and the metal-ligand stretching frequency.

The FeOEPBr spectrum features a mode at 274 cm'l, Figure 8, which is

close to the reported value for 0(Fe-Br) at 270 cm-16

. Although there are other
porphyrin vibrations in this region, as shown by the presence of a split mode
at 245 and 270 cm'1 in the spectra of other derivatives, the single 274 cm'1
band of the bromo adduct most likely reflects the 0(Fe-Br) as well. 54F e
substitution should confirm this assignment. The feature at 245 cm'1 in
FeOEPI spectrum may not be due to 0(Fe-I) reported at 246 cm-16 owing to the
presence of a similar mode at 246 cm'1 in the spectrum of the chloro

derivative, Figure 8. The stretching force constants in the ferric halide

complexes differ in the order K(Fe-F)> K(Fe-Cl)> K(Fe-Br), as reported by

152

 

>.:mzm:.ZH Z<2<m

 

RAMAN SHIFT (cm")

FeOEPCI
benzene

I
h
o 5
0
I

X" -4I3.lnm

   

.nonl

>r_._m2m._.ZH Z<E<m

I

g

 

%

200 300 400 500 600 700 800 900

RAMAN SHIFT (cm")

Figure 7. (a) R spectra of 54/56FeOEPF ; (b) R

spectra of 54/56FeOEPCl .

153

)‘ex 9 406.7 nm

Fe(III)OEPx
benzene

 

 

 

    

sun: I

 

 

 

 

>tm2m._.ZH Z<§<m

 

eoo l000 (zoo
FREQUENCY (cm")

600

400

 

 

200

The solvent bands

Figure 8. RR spectra of FeOEPX .

are marked with an asterisk.

154

Kitagawa 111.14 Meso-deuteration, 54./56Fe, as well as 15N-OEP shifts will be
utilized to make definite assignments.

We have not yet. observed the 0(M-X) in the oxidized species. It is
possible, however, that L—> M CT transitions are red-shifted or more likely
vanished in the n-cation radicals. Since there are no differences in the
absorption spectra of ferric porphyrin radicals, Figure 4, as there are in the
corresponding neutral species, Figure 3, the CT transitions, based on uv-vis
features, are expected not to be present or to be of very low probability. On the
other hand, the cobaltic porphyrin radical for X = Br reveals a feature at ~ 160
cm'1 with 676.4 nm RR excitation (not shown) which we believe to be a mode
involving metal and the bromide ligand.16 This mode is also observed in the
spectrum excited at 406.7 nm (not shown), but not at 363.8 nm, Figure 9.

The optical spectra of cobaltic-alkyl complexes differ substantially from
those of six-coordinate Co(ROH)20EPX and five-coordinate CoOEPX: the
absorption maxima of the Soret,[3 and 0: bands hardly shifts relative to those
of CoOEP and remain at 390, 520 and 552 nm, respectively. The characteristic
optical feature, however, is the nearly equal intensity of the a and [3 bands.9

CoOEPR (where R = Me, Et, i-Pr and t-Bu) displays an infrared mode at
343, 344, 339 and 343 cm‘l, respectively, analogous to that observed in
methylcobalamine at 348 cm-1.17 The latter absorption was shown to be due
to the 0(Co-C) by the slow disappearance of this band when the sample (KBr
pallet) was irradiated with tungsten light: the photolysis product,
hydroxocobalamine does not absorb at this frequency. The absence of any
significant changes in the 0(Co-C) of these complexes may be the result of
offsetting effects. The increase in the steric bulk of the alkyl chain, which

tends to decrease the bond frequency by virtue of mass effect, is compensated

155

OVQNI

 

 

ZBr'

 

 

 

 

 

 

 

 

 

 

>._._mZMPZH 2325”.

I000 |200 I400 I600

 

200 400 600 800

FREQUENCY (cm")

Figure 9. RR spectra of ComOEP+°2X' i

(a) X' =

FeCl4' ~ C1”; (b) X' = Br‘.

156

by the greater electron-releasing ability of the more substituted alkyl group,
which increases the force constant, bond order and bond frequency of the Co-
C bond. Further confirmation of this mode requires isotopic substitution of
the alkyl chain as well as preparation of Co-R derivatives of other porphyrins.
Besides, a more complete investigation of 1)(CoC) measurement should also
include other macrocyclic ligands such as chlorins and corrins to develop a

better understanding of the use of the corrin ring in vitamin B12 prosthetic

group, as a means of modulation of the Co-C bond strength.

10.

11.

REFERENCES

Buchler, I. W.; Kokisch, W.; Smith, P. D., Struct. Bonding 1978, 84, 79.

Su, Y. 0.; Czernuszerwics, R. 5.; Miller, L. A.; Spiro, T. G., I. Am. Chem.
Soc., 1988, 1_1_0_, 4150.

(a) O'Keefe, D. H.; Barlow, C. H.; Smythe, G. A.; Fuchsman, W. H.; Moss,
T. H.; Lilienthal, H. R; Caughy, W. 5., Bioinorg. Chem., 1975, 8, 125.

(b) Lueken, H.; Buchler, I. W.; Lay, K. L.; Z. N aturforsh, 1976, 3_112, 1596.

(c) Vaska, L.; Amundsen, a. R.; Brady, R.; Flynn, B. R.; Nakai, H., Finn.
Chem. Lett., 1974, 66.

(d) Ogoshi, H.; Watanabe, E.; Yoshida, Z.; Kincaid, I.; Nakamoto, K., I.
Am. Chem. Soc., 1973, g, 2845. '

(e) Strauss, S. H.; Pawlik, M. I.; Skowyra, I.; Kennedy, I. R.; Anderson, 0.
P.; Spartalian, K.; Dyne, I. L., Inorg. Chem. 1987, 26, 724.

Hashimoto, 5.; Tatsuno, Y.; Kitagawa, T., I. Am. Chem. Soc., 1987,109,
8096.

Geno, M. K.; Halpern, I., I. Am. Chem. Soc., 1987, 102, 1238.

Ogoshi, H.; Watanabe, E.; Yoshida, Z.; Kincaid, I.; Nakamoto, K., I. Am.
Chem. Soc., 1973, 22, 2845.

Choi, 5.; Spiro, T. G.; I. Am. Chem. Soc., 1983, 186, 3683.
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Ogoshi, H.; Watanabe, E. I.; Kokatsu, N .; Yoshida, Z. 1., Bull. Chem. Soc.
Ipn., 1976, Q, 2529.

Setsune, I. I.; Ikeda, M.; Kishimoto, Y.; Kitao, T.; I. Am. Chem. Soc., 1986,
188, 1309.

(a) Tait, C. D.; Holten, D.; Gouterman, M., I. Am. Chem. Soc., 1984, 106,
6653.

(b) Tait, C. D.; Holten, D.; gouterman, M., Chem. Phys. Lett., 1983, 100,
268.

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13.

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15.

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17.

18.

158

Edwards, W. D.; Zerner, M. 0, Can. I. Chem., 1985, 68, 1763.

Hendrickson, D. N .; Kinnaird, M. G.; Suslick, K. 5., I. Am. Chem. Soc.,
1987, 122, 1243.

Kitagawa, T.; Abe, M.; Kyogoku, Y.; Ogoshi, H.; Watanabe, E.; Yoshida, Z.,
I. Phys. Chem., 1976, 8_0, 1181.

Kincaid, I.; N akamoto, K., Spectros. Lett., 1976, 2, 19.

Oertling, W. A.; Salehi, A.; Chung, Y. C.; Leroi, G. E.; Chang, C. K.;
Babcock, G. T., I. Phys. Chem., _91, 5887.

Hogenkamp, H. P. 0; Rush, I. E.; Swenson, C. A., I. Biol. Chem., 1965, Lg,
3641.

(a) N akamoto, K., In "Infrared and Raman Spectra of Inorganic
Coordination Compounds", Nakamoto, K., Ed., Iohn Wiley 8: Sons:
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(b) Clark, R I.; Williams, C. 5., Inorg. Chem., 1965, 4, 350.

(c) Nakagawa, I.; Shimanouchi, T., Spectrochim. Acta, 1966, 2_2_, 759.

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(e) Ref. 18a, p. 379.

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